mail/postsrsd: Remove USE_LDCONFIG

postsrsd does not install shared libraries but the port uses
USE_LDCONFIG causing the following warning:

ldconfig: warning: /usr/local/lib/postsrsd: No such file or directory

Remove USE_LDCONFIG to compensate

PR: 211097
Reported by: Miroslav Lachman <000.fbsd quip cz>
Approved by: Krzysztof <ports bsdserwis com> (maintainer)
Approved by: portmgr (blanket)
MFH: 2016Q3