Copter: fully honour FENCE_OPTION to disable mode changes

when the user has chosen to disallow mode change during fence breach
it should be fully implemented, without a landing exception.

as requested by Pete, and discussed on dev call

ArduCopter/mode.cpp

@@ -353,7 +353,6 @@ bool Copter::set_mode(Mode::Number mode, ModeReason reason)
         fence.enabled() &&
         fence.option_enabled(AC_Fence::OPTIONS::DISABLE_MODE_CHANGE) &&
         fence.get_breaches() &&
-        !flightmode->is_landing() &&
         motors->armed() &&
         get_control_mode_reason() == ModeReason::FENCE_BREACHED &&
         !ap.land_complete) {