Revert "[core] support .gnu.linkonce sections"

This reverts commit 5d5ca72a7876c3dd3dd1db83dc6a0d74be9e2cd1.

Discuss on a better design to raise error when there is a similar group with Gnu
linkonce sections and COMDAT sections.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@205224 91177308-0d34-0410-b5e6-96231b3b80d8