update navigation_tools.md to remove links
update navigation_tools.md to remove links

CLOSES: PLAT-3996

Test plan:
- run bundle exec rake doc:api
- open navigation tools page (under external tools)
- verify the page contains no links

Change-Id: Icfed5e239037783e25b8acb192e9a7d5b4909f55
Reviewed-on: https://gerrit.instructure.com/171855
Tested-by: Jenkins
QA-Review: Erin Hallmark <2a4b17b11682b229726079a631360cf016a43450@instructure.com>
Product-Review: Erin Hallmark <2a4b17b11682b229726079a631360cf016a43450@instructure.com>
Reviewed-by: Weston Dransfield <e9203059795dc94e28a8831c65eb7869e818825e@instructure.com>

instructure/canvas-lms,instructure/canvas-lms,sfu/canvas-lms,djbender/canvas-lms,sfu/canvas-lms,sfu/canvas-lms,instructure/canvas-lms,instructure/canvas-lms,djbender/canvas-lms,sfu/canvas-lms,instructure/canvas-lms,instructure/canvas-lms,sfu/canvas-lms,instructure/canvas-lms,sfu/canvas-lms,djbender/canvas-lms,djbender/canvas-lms,sfu/canvas-lms