[mms-engine] Avoid using qmake to detect Qt version

qtchooser systems will only have a default choice for qmake when a default package is installed, which we can't depend on. This causes build failures, see: https://build.merproject.org/package/rawlog?arch=armv7el&package=mms-engine&project=nemo%3Adevel%3Amw&repository=latest_armv7l

There is no reason to keep Qt4 support here.