[lldb][ClangExpressionParser] Remove duplicate construction of ExternalASTSourceWrapper

This is an oversight from #104817 where the intention
was to hoist the ExternalASTSourceWrapper construction out of the
conditional so it can be set on both the `SemaSourceWithPriorities` and
be added as an external source to Sema. But the inner
`ExternalASTSourceWrapper` allocation wasn't actually removed.

This currently all works fine because all these AST sources are
refcounted and point to the same underlying AST sources. But this
patch cleans this up regardless.

Author: Michael137

lldb/source/Plugins/ExpressionParser/Clang/ClangExpressionParser.cpp

@@ -1229,8 +1229,6 @@ ClangExpressionParser::ParseInternal(DiagnosticManager &diagnostic_manager,
       auto *module_wrapper =
           new ExternalASTSourceWrapper(ast_context.getExternalSource());
 
-      auto *ast_source_wrapper = new ExternalASTSourceWrapper(ast_source);
-
       auto *multiplexer =
           new SemaSourceWithPriorities(module_wrapper, ast_source_wrapper);