Modules: Clean up ModuleFile::Imports in ModuleManager::removeModules

I don't have a testcase for this (and I'm not sure if it's an observable
bug), but it seems obviously wrong that ModuleManager::removeModules is
failing to clean up deleted modules from ModuleFile::Imports.  See the
code in ModuleManager::addModule that inserts into ModuleFile::Imports;
we need the inverse operation.

llvm-svn: 293399