Revert "'use utf8' should imply /u regex matching"

This reverts commit bfa0ee78b652802412c3cab86bb873ed67ea6550.

This commit turned out to be contentious, and since we are past the
contentious features freeze date, no matter what else, it should be
reverted.

The argument for the commit essentially boils down to 'use utf8'
indicates that the text within its scope should be treated as utf8.
That means that any patterns with literals in them should be treated as
utf8, but utf8-encoded patterns follow Unicode rules by definition.

The arguments against it are that code relies on the way it has always
worked (even if that was an oversight), and in fact several CPAN modules
were broken by it, [perl #121162].  Also it has been the stated intent
that 'use utf8' will eventually become a no-op, meaning all text will be
treated as utf8, and that shouldn't have to mean that backwards
compatibility will be broken then.