discussions: remove extraneous screenreader label on search
discussions: remove extraneous screenreader label on search

fixes CNVS-6653

test plan:
  * open a discussion topic
  * verify with your browser's DOM inspector that there is
      not a hidden label for the topic search box as well
      as an aria-label

Change-Id: I5c6a3c3089a3e71f96ad72f8869356bd3ee10358
Reviewed-on: https://gerrit.instructure.com/24395
Tested-by: Jenkins <d95b56ce41a2e1ac4cecdd398defd7414407cc08@instructure.com>
Reviewed-by: Joel Hough <4816dabf8db1bc6cac35b3a24cab2ff844b5b0c7@instructure.com>
QA-Review: Cam Theriault <b4133c36b6d893d0872cc09e87b536d16aa2d241@instructure.com>
Product-Review: Braden Anderson <a6cc2cef7d25f27847876bdb81651755fcebacf0@instructure.com>

samuels410/greatlakes-lms,venturehive/canvas-lms,leftplusrightllc/canvas-lms,jay3126/canvas-lms,kyroskoh/canvas-lms,va7map/canvas-lms,SwinburneOnline/canvas-lms,venturehive/canvas-lms,skmezanul/canvas-lms,va7map/canvas-lms,ShreniiNepal/shrenii_lmp,snehakachroo1/LSI_replica,redconfetti/canvas-lms,roxolan/canvas-lms,instructure/canvas-lms,matematikk-mooc/canvas-lms,Rvor/canvas-lms,fronteerio/canvas-lms,antoniuslin/canvas-lms,wimemx/Canvas,wrdsb/canvas-lms,jay3126/canvas-lms,Jyaasa/canvas-lms,Unow/canvas-lms,juoni/canvas-lms,wimemx/Canvas,gbleydon/canvas-lms,wrdsb/canvas-lms,Jyaasa/canvas-lms,efrj/canvas-lms,wye0220/shindig-canvas,wye0220/shindig-canvas,roxolan/canvas-lms,shidao-fm/canvas-lms,shidao-fm/canvas-lms,greyhwndz/canvas-lms,instructure/canvas-lms,narigone/canvas,arrivu/arrivu-lms,grahamb/canvas-lms,leftplusrightllc/canvas-lms,sfu/canvas-lms,utkarshx/canvas-lms,landonwilkins/canvas-lms,djbender/canvas-lms,va7map/canvas-lms,kyroskoh/canvas-lms,dgynn/canvas-lms,eCNAP/CANVAS,arrivu/qibla-ecampus,greyhwndz/canvas-lms,wye0220/shindig-canvas,samuels410/greatlakes-lms,AndranikMarkosyan/lms,redconfetti/canvas-lms,HotChalk/canvas-lms,whalejasmine/canvas-lms,Unow/canvas-lms,eCNAP/CANVAS,whalejasmine/canvas-lms,venturehive/canvas-lms,arrivu/qibla-ecampus,antoniuslin/canvas-lms,AndranikMarkosyan/lms,tgroshon/canvas-lms,gbleydon/canvas-lms,sfu/canvas-lms,matematikk-mooc/canvas-lms,instructure/canvas-lms,sfu/canvas-lms,redconfetti/canvas-lms,antoniuslin/canvas-lms,grahamb/canvas-lms,roxolan/canvas-lms,matematikk-mooc/canvas-lms,djbender/canvas-lms,usmschuck/canvas,fronteerio/canvas-lms,juoni/canvas-lms,instructure/canvas-lms,shidao-fm/canvas-lms,tgroshon/canvas-lms,usmschuck/canvas,shidao-fm/canvas-lms,sfu/canvas-lms,djbender/canvas-lms,sfu/canvas-lms,roxolan/canvas-lms,sdeleon28/canvas-lms,arrivu/hoodemo,snehakachroo1/LSI_replica,dgynn/canvas-lms,whalejasmine/canvas-lms,landonwilkins/canvas-lms,sfu/canvas-lms,fronteerio/canvas-lms,samuels410/greatlakes-lms,sdeleon28/canvas-lms,sdeleon28/canvas-lms,leftplusrightllc/canvas-lms,loganrun/pincan,tgroshon/canvas-lms,arrivu/hoodemo,efrj/canvas-lms,instructure/canvas-lms,instructure/canvas-lms,grahamb/canvas-lms,SwinburneOnline/canvas-lms,HotChalk/canvas-lms,kyroskoh/canvas-lms,wimemx/Canvas,efrj/canvas-lms,sfu/canvas-lms,loganrun/pincan,ShreniiNepal/shrenii_lmp,leftplusrightllc/canvas-lms,arrivu/qibla-ecampus,juoni/canvas-lms,instructure/canvas-lms,juoni/canvas-lms,utkarshx/canvas-lms,whalejasmine/canvas-lms,wrdsb/canvas-lms,SwinburneOnline/canvas-lms,Jyaasa/canvas-lms,va7map/canvas-lms,jay3126/canvas-lms,HotChalk/canvas-lms,arrivu/hoodemo,wrdsb/canvas-lms,Rvor/canvas-lms,HotChalk/canvas-lms,greyhwndz/canvas-lms,skmezanul/canvas-lms,grahamb/canvas-lms,landonwilkins/canvas-lms,utkarshx/canvas-lms,loganrun/pincan,dgynn/canvas-lms,arrivu/arrivu-lms,venturehive/canvas-lms,sdeleon28/canvas-lms,usmschuck/canvas,kyroskoh/canvas-lms,fronteerio/canvas-lms,Unow/canvas-lms,utkarshx/canvas-lms,grahamb/canvas-lms,ShreniiNepal/shrenii_lmp,Rvor/canvas-lms,skmezanul/canvas-lms,SwinburneOnline/canvas-lms,djbender/canvas-lms,landonwilkins/canvas-lms,antoniuslin/canvas-lms,eCNAP/CANVAS,eCNAP/CANVAS,arrivu/arrivu-lms,ShreniiNepal/shrenii_lmp,narigone/canvas,Jyaasa/canvas-lms,efrj/canvas-lms,jay3126/canvas-lms,snehakachroo1/LSI_replica,Rvor/canvas-lms,snehakachroo1/LSI_replica,narigone/canvas,redconfetti/canvas-lms,matematikk-mooc/canvas-lms,narigone/canvas,gbleydon/canvas-lms,AndranikMarkosyan/lms,samuels410/greatlakes-lms,dgynn/canvas-lms,tgroshon/canvas-lms,greyhwndz/canvas-lms,usmschuck/canvas,AndranikMarkosyan/lms,skmezanul/canvas-lms