etcupdate: remove redundant diff3 flag

-A and -m select different output modes output modes for diff3. When
both flags are present gnu diff3 prefers -m, drop the extra -A flag in
etcupdate.

Reviewed by: pstef, 0mp
MFC after: 3 days
Sponsored by: Klara Inc
Differential Revision: https://reviews.freebsd.org/D###