Retire GNU diff3

We added the option to build BSD diff3 in commit 2201f7c49f11 ("Build
BSD diff3 if GNU diff3 is disabled.") and made it the default in
4d5c434ed16e ("diff3: use bsd diff3 by default") after resolving
deficiencies relative to GNU diff3. Thus, we can now remove the GNU
diff3 build infrastructure and source.

Reviewed by: bapt, ziaee
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D46878