[gardening] Remove unused LazySKDUID RequestMangleSimpleClass

Introduced in commit 6cbcb7b

Author: practicalswift

tools/SourceKit/tools/sourcekitd/lib/API/Requests.cpp

@@ -61,7 +61,6 @@ class LazySKDUID {
 static LazySKDUID RequestProtocolVersion("source.request.protocol_version");
 
 static LazySKDUID RequestDemangle("source.request.demangle");
-static LazySKDUID RequestMangleSimpleClass("source.request.mangle_simple_class");
 
 static LazySKDUID RequestIndex("source.request.indexsource");
 static LazySKDUID RequestDocInfo("source.request.docinfo");