Remove unnecessary --warn-shared-textrel flags.

This flag has never done anything for ld.lld, since it has this behavior
by default (and actually can't turn it off either). It's now being
removed as a valid flag from ld.lld, so it's time for Android to drop
the unnecessary flags. See https://reviews.llvm.org/D118942.

Test: builds
Change-Id: I22c344b7a55e275c1fbd5fef25f85e97a39de5c4