devel/utf8cpp3: Remove obsoleted port

Use devel/utf8cpp instead.

devel/utf8cpp3 was created for textproc/apertium as it was the only port which do not work with utf8cpp 4.*.
It is no longer needed after 04e8d6e30a276fe64a23259406c6fe905f58044c.

PR: 284627
Reported by: diizzy
Reference: https://cgit.freebsd.org/ports/commit/?id=18e703731c1b9e6b1ee31866ea9a83d904a701aa

		https://cgit.freebsd.org/ports/commit/?id=04e8d6e30a276fe64a23259406c6fe905f58044c