memcpy.3: remove BUGS section allowing overlapping strings

The removed text claimed that memcpy is implemented using bcopy and thus
strings may overlap. Use of bcopy is an implementation detail that is
no longer true, even if the implementation (on some archs) does allow
overlap.

In any case behaviour is undefined per the C standard if memcpy is
called with overlapping objects, and this man page already claimed that
src and dst may not overlap.

Reviewed by: kib
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D31192