Removing an unused function; NFC

It turns out there are zero in-tree callers of CallExpr::getNumCommas()
so it's reasonable to remove.

clang/include/clang/AST/Expr.h

@@ -3061,10 +3061,6 @@ public:
                               PREARGS_START + getNumPreArgs() + getNumArgs());
   }
 
-  /// getNumCommas - Return the number of commas that must have been present in
-  /// this function call.
-  unsigned getNumCommas() const { return getNumArgs() ? getNumArgs() - 1 : 0; }
-
   /// Get FPOptionsOverride from trailing storage.
   FPOptionsOverride getStoredFPFeatures() const {
     assert(hasStoredFPFeatures());