Eliminates $?ENC

Str.encode does not seem to have ever used it, even going back in
history 7 years:
https://github.com/rakudo/rakudo/blame/8453ed1c076bda52161ddd792ee8dd81df5790e3/src/core/Str.pm
I haven't checked the other mentioned, but it does not seem to be
either in Rakudo or roast now. Refs #2547

doc/Language/variables.pod6

@@ -1150,7 +1150,6 @@ The following compile time variables allow for a deeper introspection:
     $?GRAMMAR        Which grammar am I in?
     $?TABSTOP        How many spaces is a tab in a heredoc or virtual margin?
     $?NL             What a vertical newline "\n" means: LF, CR or CRLF
-    $?ENC            Default encoding of various IO methods, e.g., Str.encode, Buf.decode
     $?DISTRIBUTION   The Distribution of the current compilation unit.
 
 With particular regard to the C<$?NL>, see the  L<newline pragma|/language/pragmas>.