svnadmin/hooks: Remove mfh hook

MFHs do not require approval anymore, so unhook the mfh email generator
from the post-commit hook.

The email did contain a helpful hint as to the command necessary to perform
the MFH, but it often required manual intervention and investigation to name
all previous revisions between the branch and head. Given how frequently the
suggested command was incomplete, it's probably better in the long-run that
committers do the process by hand.

Reviewed by: bapt
Differential Revision: https://reviews.freebsd.org/D27514