Drop all the unnecessary "use utf8" clauses and some of

Drop all the unnecessary "use utf8" clauses and some of
the unnecessary "use bytes" ones.

TODO: scour the documentation for unnecessary "use utf8"
and prominently display it in perldelta when the time comes.
("use utf8" should be necessary ONLY if one wants the script
to be in UTF-8.)  Also should be checked in some non-ASCII
non-Latin-1 platform, like EBCDIC.

p4raw-id: //depot/perl@11638