Delete the MANLINT variable and the related SUFFIXES rules because

Delete the MANLINT variable and the related SUFFIXES rules because
since yesterday, "mandoc -Tlint -Wfatal" can no longer fail.
Instead, as suggested by deraadt@, provide a manlint target
that is *not* run during make build, but can be run
whenever you want to check syntax of manuals.
"nice stuff" deraadt@