[PECOFF] Remove unnecessary assertion.

Bugs that would be caught by this assertion would also be caught by
RoundTripYAMLPass test. We've enabled the pass for PECOFF, so we can remove
this.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@193886 91177308-0d34-0410-b5e6-96231b3b80d8