Remove $!unit_ready

It's replaced with `$!setting_loaded` with somewhat different semantics.
Since `$!unit_ready` was used only once and not exposed to the public
it's now safe to remove it.

src/Perl6/World.nqp

@@ -516,7 +516,6 @@ class Perl6::World is HLL::World {
     has $!setting_fixup_task;
 
     has int $!in_unit_parse;
-    has int $!unit_ready;
     has int $!have_outer;
     has int $!setting_loaded;
     has $!setting_name;
@@ -526,7 +525,6 @@ class Perl6::World is HLL::World {
         %!code_object_fixup_list := {};
         $!record_precompilation_dependencies := 1;
         %!quote_lang_cache := {};
-        $!unit_ready := 0;
         $!setting_loaded := 0;
         $!in_unit_parse := 0;
     }
@@ -829,8 +827,6 @@ class Perl6::World is HLL::World {
             }
         }
 
-        $!unit_ready := 1;
-
         self.add_load_dependency_task(:deserialize_ast($!setting_fixup_task), :fixup_ast($!setting_fixup_task));
     }