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.

tests/regex/wxregextest.cpp

@@ -184,8 +184,6 @@ TEST_CASE("wxRegEx::ConvertFromBasic", "[regex][basic]")
     CHECK( wxRegEx::ConvertFromBasic("[^$\\)]") == "[^$\\)]" );
 }
 
-#ifdef wxHAS_REGEX_ADVANCED
-
 TEST_CASE("wxRegEx::Unicode", "[regex][unicode]")
 {
     const wxString cyrillicCapitalA(L"\u0410");
@@ -198,6 +196,4 @@ TEST_CASE("wxRegEx::Unicode", "[regex][unicode]")
     CHECK( re.GetMatch(cyrillicSmallA) == cyrillicSmallA );
 }
 
-#endif // wxHAS_REGEX_ADVANCED
-
 #endif // wxUSE_REGEX