Simplify plane selection for drop, fly, recon and sweep

Simplify plane selection for drop, fly, recon and sweep

Plane flying commands first select the planes to fly the mission and
their escorts, then equip them.  They all fail when no planes to fly
the mission can be equipped.

Unlike bomb and paradrop, commands drop, fly, recon and sweep had an
additional check that made them fail when no planes to fly the mission
could be selected.  Because "none selected" implies "none equipped",
the additional check is redundant.  Remove it.

While there, break lines in calls of pln_sel() more tastefully.