Removed tabindex="-1" from main element to fix accessibility issue
Description
Removed tabindex="-1" from main
We had a strange issue where TalkBack wasn’t reading content in some xBlocks but was reading some others. I figured out that if you remove tabindex=-1 from <main>, TalkBack works as expected again. Tangentially, this breaks the old method of implementing SkipNav links. Further tangentially, the SkipNav links were already broken (or in this case redundant on mobile). @wittjeff will file two separate issues after this one PR is merged — one for a better way to implement SkipNav links for Learning MFE, and one for a better / functional way to implement SkipNav links for Mobile.
How to reproduce
Go to [this unit](https://courses.edx.org/xblock/block-v1:W3Cx+WAI0.1x+3T2019+type@html+block@2eb3c86c479f44ba964f88ff0bfd9211) on mobile and turn on TalkBack. It will skip main content. For further information on testing go to [Jira Ticket](https://openedx.atlassian.net/browse/LEARNER-7858)
186cc9f16c