Remove disable_lua51_features config option

In practice, we're never going to change this to true by default. The
old Tekkit Legends pack enabled this[^1], and that caused a lot of
problems, though admittedly back in 2016 so things might be better now.

If people do want this functionality, it should be fairly easy to
replicate with a datapack, adding a file to rom/autorun.

[^1]: See https://www.computercraft.info/forums2/index.php?/topic/27663-

      Hate that I remember this, why is this still in my brain?

projects/web/src/main/java/dan200/computercraft/core/TCoreConfig.java

@@ -16,7 +16,6 @@ public final class TCoreConfig {
     }
 
     public static int maximumFilesOpen = 128;
-    public static boolean disableLua51Features = false;
     public static String defaultComputerSettings = "";
 
     public static boolean httpEnabled = true;