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/core/src/main/java/dan200/computercraft/core/CoreConfig.java

@@ -22,7 +22,6 @@ public final class CoreConfig {
     }
 
     public static int maximumFilesOpen = 128;
-    public static boolean disableLua51Features = false;
     public static String defaultComputerSettings = "";
 
     public static boolean httpEnabled = true;

projects/core/src/main/java/dan200/computercraft/core/lua/CobaltLuaMachine.java

@@ -78,7 +78,6 @@ public class CobaltLuaMachine implements ILuaMachine {
         Bit32Lib.add(state, globals);
         globals.rawset("_HOST", ValueFactory.valueOf(environment.hostString()));
         globals.rawset("_CC_DEFAULT_SETTINGS", ValueFactory.valueOf(CoreConfig.defaultComputerSettings));
-        if (CoreConfig.disableLua51Features) globals.rawset("_CC_DISABLE_LUA51_FEATURES", Constants.TRUE);
 
         // Add default APIs
         for (var api : environment.apis()) addAPI(globals, api);

projects/core/src/main/resources/data/computercraft/lua/bios.lua

@@ -42,31 +42,20 @@ if _VERSION == "Lua 5.1" then
         end
     end
 
-    if _CC_DISABLE_LUA51_FEATURES then
-        -- Remove the Lua 5.1 features that will be removed when we update to Lua 5.2, for compatibility testing.
-        -- See "disable_lua51_functions" in ComputerCraft.cfg
-        setfenv = nil
-        getfenv = nil
-        loadstring = nil
-        unpack = nil
-        math.log10 = nil
-        table.maxn = nil
-    else
-        loadstring = function(string, chunkname) return nativeload(string, chunkname) end
-
-        -- Inject a stub for the old bit library
-        _G.bit = {
-            bnot = bit32.bnot,
-            band = bit32.band,
-            bor = bit32.bor,
-            bxor = bit32.bxor,
-            brshift = bit32.arshift,
-            blshift = bit32.lshift,
-            blogic_rshift = bit32.rshift,
-        }
-    end
+    loadstring = function(string, chunkname) return nativeload(string, chunkname) end
 end
 
+-- Inject a stub for the old bit library
+_G.bit = {
+    bnot = bit32.bnot,
+    band = bit32.band,
+    bor = bit32.bor,
+    bxor = bit32.bxor,
+    brshift = bit32.arshift,
+    blshift = bit32.lshift,
+    blogic_rshift = bit32.rshift,
+}
+
 -- Install lua parts of the os api
 function os.version()
     return "CraftOS 1.8"