Localise counting of primary partitions into max_amount_prim_reached()

Now that display_partitions is a Win_GParted member variable and
therefore available throughout the class, since commit [1], calculation
of primary_count can be localised in max_amount_prim_reached() where it
is used.

Implements a FIXME and removes primary_count as a member variable.

[1] 545b75d9
    Move vector of partition objects to a Win_GParted class member (#750168)