Remove skipFunctionArguments() and code only it used

This function was used only in KDevelop's old custom C++ parser
(oldcpp) until its removal in f31d32f4.

This function contains one of two uses of another function
findCommaOrEnd(). Remove this dead usage in order to modify
findCommaOrEnd() without worrying about it.

Also remove a simple helper class SafetyCounter and 3 helper functions
declared in stringhelpers.cpp, which were used only in
skipFunctionArguments().