Drop WX_NO_REGEX_ADVANCED and always define wxHAS_REGEX_ADVANCED

Using non-PCRE system regex library, which was the only build variant
when wxHAS_REGEX_ADVANCED was not defined, is not supported any more, so
simplify the code by not testing for it.

include/wx/regex.h

@@ -27,10 +27,8 @@ enum
     // use extended regex syntax
     wxRE_EXTENDED = 0,
 
-    // use advanced RE syntax (built-in regex only)
-#ifdef wxHAS_REGEX_ADVANCED
+    // use advanced RE syntax (deprecated, same as wxRE_EXTENDED now)
     wxRE_ADVANCED = 1,
-#endif
 
     // use basic RE syntax
     wxRE_BASIC    = 2,