LibCore: Remove the no-longer-used Core::DateTime::is_before() function

Author: https://github.com/alimpfard
Commit: db8f0a2fa6
Pull-request: https://github.com/SerenityOS/serenity/pull/6443
Reviewed-by: https://github.com/awesomekling

Userland/Libraries/LibCore/DateTime.cpp

@@ -249,10 +249,4 @@ String DateTime::to_string(const String& format) const
     return builder.build();
 }
 
-bool DateTime::is_before(const String& other) const
-{
-    auto now_string = String::formatted("{:04}{:02}{:02}{:02}{:02}{:02}Z", year(), month(), weekday(), hour(), minute(), second());
-    return __builtin_strcasecmp(now_string.characters(), other.characters()) < 0;
-}
-
 }