fix(test/string): avoid use of deleted function

Remove portion of code to fix a build issue for gcc16(C++23), where the build fails due to test `error: use of deleted function ‘std::basic_ostream<char, _Traits>& std::operator<<(basic_ostream<char, _Traits>&, const wchar_t*) [with _Traits = char_traits<char>]’
  148 |         DLIB_TEST_MSG( (left_substr(ws,L".") == L"file"), L"");`

Author: ykshek

dlib/test/string.cpp

@@ -143,13 +143,6 @@ namespace
         DLIB_TEST( (left_substr(a,"") == "file.txt"));
         DLIB_TEST( (right_substr(a,"") == ""));
 
-        wstring ws = L"file.txt";
-        DLIB_TEST( (left_substr(ws,wstring(L".")) == L"file"));
-        DLIB_TEST_MSG( (left_substr(ws,L".") == L"file"), L"");
-        DLIB_TEST( (right_substr(ws,wstring(L".")) == L"txt"));
-        DLIB_TEST_MSG( (right_substr(ws,L".") == L"txt"), L"");
-
-
         dlog << LTRACE << 8;
         {
             ostringstream sout;