[ELF][Cleanup] Remove unused code.

The old code was used as a workaround to fix how relocations are calculated for
sections with SHF_MERGE|SHF_STRINGS attribute. This patch removes the erroneous
code.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@220159 91177308-0d34-0410-b5e6-96231b3b80d8