Revert "Reland D64327 [MC][ELF] Allow STT_SECTION referencing SHF_MERGE on REL targets"

This reverts commit b497665d.

Spent some time trying to reproduce this locally, reverting in a
desparate attempt to fix the sanitizer buildbot:
 - http://lab.llvm.org:8011/builders/sanitizer-x86_64-linux/builds/28828

I don't know exactly why or how this patch breaks the bots, but it seems
pretty concrete that it's the culprit.