libexec/diff3prog: Don't duplicate the installation of diff3.1

The diff3.1 man page is already being installed by usr.bin/diff3.

world - Replace gnu diff, diff3, and sdiff with BSD versions

* Replace diff and diff3 with the OpenBSD diff and diff3,
Replace sdiff with the FreeBSD sdiff,
Remove the GNU diff, diff3, and sdiff

world - Replace gnu diff, diff3, and sdiff with BSD versions

* Replace diff and diff3 with the OpenBSD diff and diff3,
Replace sdiff with the FreeBSD sdiff,
Remove the GNU diff, diff3, and sdiff

* Basic testing with dsynth bulk to ensure no significant compatibility
disruptions.

Submitted-by: Tyler <tyler.clark@foxide.xyz>

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