[PECOFF] Do not accept "/include <string>" option.

Command line argument separator on Windows is colon. We used to accept space
as a separator, but we removed most of the code for that. This patch is to
delete the remaining code.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@198032 91177308-0d34-0410-b5e6-96231b3b80d8