[lldb][ClangASTImporter][NFC] Remove redundant calls to ASTImporter::Imported

The ASTImporter::Imported base method has been made a no-op in
26f72a96. So all calls to it from
a base-class are now redundant. The API is now only used to notify
subclasses that an import occurred and not for any other
bookkeeping (which is done in MapImported which we call properly).

Differential Revision: https://reviews.llvm.org/D158172