remove article.keywords from simple theme
remove article.keywords from simple theme

We don't process 'keywords' metadata specially, so it never gets
processed into a list.

This prevents issues like #1733.

rbarraud/pelican,simonjj/pelican,iKevinY/pelican,ehashman/pelican,51itclub/pelican,jimperio/pelican,getpelican/pelican,simonjj/pelican,karlcow/pelican,garbas/pelican,number5/pelican,lucasplus/pelican,gymglish/pelican,abrahamvarricatt/pelican,GiovanniMoretti/pelican,JeremyMorgan/pelican,GiovanniMoretti/pelican,ingwinlu/pelican,talha131/pelican,ehashman/pelican,zackw/pelican,catdog2/pelican,ingwinlu/pelican,abrahamvarricatt/pelican,Rogdham/pelican,UdeskDeveloper/pelican,Summonee/pelican,JeremyMorgan/pelican,Summonee/pelican,zackw/pelican,Scheirle/pelican,lucasplus/pelican,douglaskastle/pelican,ls2uper/pelican,number5/pelican,UdeskDeveloper/pelican,farseerfc/pelican,avaris/pelican,rbarraud/pelican,JeremyMorgan/pelican,lazycoder-ru/pelican,zackw/pelican,Rogdham/pelican,farseerfc/pelican,avaris/pelican,lucasplus/pelican,Scheirle/pelican,lazycoder-ru/pelican,talha131/pelican,karlcow/pelican,51itclub/pelican,UdeskDeveloper/pelican,gymglish/pelican,florianjacob/pelican,douglaskastle/pelican,GiovanniMoretti/pelican,lazycoder-ru/pelican,Scheirle/pelican,ls2uper/pelican,getpelican/pelican,garbas/pelican,gymglish/pelican,rbarraud/pelican,karlcow/pelican,Summonee/pelican,florianjacob/pelican,number5/pelican,douglaskastle/pelican,51itclub/pelican,catdog2/pelican,iKevinY/pelican,simonjj/pelican,ls2uper/pelican,ehashman/pelican,florianjacob/pelican,abrahamvarricatt/pelican,jimperio/pelican,catdog2/pelican,jimperio/pelican,Rogdham/pelican,garbas/pelican