Don't inherit mixin class language revision
Don't inherit mixin class language revision

Use the compiler-default. This fixes #3998 and makes the following code
work:

    use v6.e.PREVIEW;
    role R { }; my $v = 0 but R;

Yet, `$v.^language-revision` will now return `e`, contrary to
`0.^language-revision` returning `c`.

tbrowder/rakudo,salortiz/rakudo,labster/rakudo,MasterDuke17/rakudo,MasterDuke17/rakudo,tony-o/rakudo,labster/rakudo,rakudo/rakudo,labster/rakudo,salortiz/rakudo,tbrowder/rakudo,tony-o/rakudo,rakudo/rakudo,rakudo/rakudo,labster/rakudo,rakudo/rakudo,MasterDuke17/rakudo,tbrowder/rakudo,tony-o/rakudo,rakudo/rakudo,tony-o/rakudo,tbrowder/rakudo,salortiz/rakudo,MasterDuke17/rakudo,tony-o/rakudo,salortiz/rakudo,tbrowder/rakudo,rakudo/rakudo,MasterDuke17/rakudo,salortiz/rakudo,tony-o/rakudo,tbrowder/rakudo,labster/rakudo,MasterDuke17/rakudo,labster/rakudo,salortiz/rakudo