Cleanup: remove str_strip_colours; StrMakeValid also removes the colours

Essentially str_strip_colours followed by StrMakeValid makes the calling of
str_strip_colours useless, as StrMakeValid would have removed them too.

src/string_func.h

@@ -42,7 +42,6 @@ void StrMakeValidInPlace(char *str, const char *last, StringValidationSettings s
 void StrMakeValidInPlace(char *str, StringValidationSettings settings = SVS_REPLACE_WITH_QUESTION_MARK);
 
 void str_fix_scc_encoded(char *str, const char *last) NOACCESS(2);
-void str_strip_colours(char *str);
 bool strtolower(std::string &str, std::string::size_type offs = 0);
 
 [[nodiscard]] bool StrValid(const char *str, const char *last) NOACCESS(2);