utf8: Remove unused variable (%utf8::enc).

In its original implementation in perl 5.6, 'use utf8' accepted
an encoding argument, which was saved in %utf8::enc and later
used by utf8_heavy.pl to load the right files from lib/unico[rd]e.

However, since 5.8 (or more specifically, 15732964418f),
utf8_heavy.pl stopped using that variable, so there's no reason
to keep it around in utf8.pm.

Bump $VERSION in lib/utf8.pm.