qbs build: Remove outdated "minimumQtVersion" specifications.

These were added when the overall project still supported Qt 4. Now the
general requirement is even stronger than what was specified in these
plugins.

Change-Id: I71241e20a72a413d30c4e089bcc19b0762a732da
Reviewed-by: Joerg Bornemann <joerg.bornemann@theqtcompany.com>