multimedia/ringrtc: Remove patch to detect mempcpy(3)

The patch was applied to detect mempcpy(3) in /usr/include/string.h. It
is no longer necessary after ports snapshot d124846683 and did not
detect the function after src snapshot 0c47b9c211.

Approved by: mikael (maintainer)
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D46058