mail/qmail-contrib: use USES=metaport

LICENSE=NONE confuses the license framework and makes it ask the user to
accept the empty license. Add USES=metaport which makes it pick up the
NA license defined therein. Then get rid of the various variables we
already set in USES=metaport.

MFH: 2024Q4.
(cherry picked from commit 990a3dcfb52e977c8c4ab2f195cf0742d03da794)