[PECOFF] Remove dead code

AddressOfEntryPoint is overridden after we layout all atoms (until then,
we don't know the entry point address for obvious reason.)
I believe this code is leftover from very early version of the
PE/COFF port that we only had an entry function in a test object file.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@222026 91177308-0d34-0410-b5e6-96231b3b80d8