net/libdnet: Drop bogus libcheck run dependency

It was added in r498511 to fix the build when devel/check is installed
in the environment. It is not used for anything other than building
a couple of tests that are never run or installed. Instead just
pass --without-check to configure to disable use of it.

PR: 237177
Reported by: Christian Ullrich <chris@chrullrich.net>