[clang] remove dead code after 2a133224

Commit 2a133224 extracted this code to a new function checkSectionName() and
added a call to it, but didn't remove the original code. The original code
is dead since the checkSectionName() early return would fire when it would
trigger. (If it weren't dead, it'd make clang crash since
err_attribute_section_invalid_for_target now takes two args instead of just the
one that's passed.)

No behavior change.

Differential Revision: https://reviews.llvm.org/D101457