[Parser] Remove "add -fmodules" from the "use of '@import' when modules are disabled, add -fmodules" error.

We disable modules for C++, at which point this is confusing.
rdar://16641918

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@206440 91177308-0d34-0410-b5e6-96231b3b80d8