Simplify the Makefile by switching to USES=qmake.

• This allows us to get rid of both manually specified targets.
• As no PLIST_SUBS are used, also OPTIONS_SUB can be dropped.
• Further, do not use QT_BINDIR and install the manpage to man/ intead of share/man.

PR: 214621
Approved by: rakuco (mentor), takefu (maintainer)