Remove copyinfrom() and copyinstrfrom().

These functions were added in 2001 and are currently unused.
copyinfrom() looks to have never been used. copyinstrfrom() was used
for two weeks before the code was refactored to remove it's sole use.

Reviewed by: brooks, kib
Obtained from: CheriBSD
Sponsored by: DARPA
Differential Revision: https://reviews.freebsd.org/D24928