Revert "[MIPS] Temporarily disable Mips shared-textrel errors"

This reverts commit 3dfd20ae - the clang
version we are using has the mentioned permanent fix. The new build
system (Soong) uses a single list of unknown clang flags for all
architectures, so workarounds like this no longer work.

Change-Id: I64a83f3b52a9ca1d11a76df5a060a36f42e27e4b