Revert "Add 'thinlto_src_module' md with asserts or -enable-import-metadata"

Reverting because of 17463
http://lab.llvm.org:8011/builders/clang-x86_64-linux-selfhost-modules/builds/17463

This reverts commit d20cb431bba2ba43b4c65a8556cff445bfefbb7c.

llvm-svn: 274946