[lldb][ASTUtils] Remove unused SemaSourceWithPriorities::addSource API

As far as I can tell, this has always been unused. My hunch is that
this was supposed to mimick the `MultiplexExternalSemaSource::AddSource`
API which `SemaSourceWithPriorities` is based on.

(cherry picked from commit c7a54bf)

Author: Michael137

lldb/source/Plugins/ExpressionParser/Clang/ASTUtils.h

@@ -295,10 +295,6 @@ class SemaSourceWithPriorities : public ImporterBackedASTSource {
 
   ~SemaSourceWithPriorities() override;
 
-  void addSource(clang::ExternalSemaSource &source) {
-    Sources.push_back(&source);
-  }
-
   //===--------------------------------------------------------------------===//
   // ExternalASTSource.
   //===--------------------------------------------------------------------===//