Removed 'degree' character that caused python3 to act up while pruning the mixer file

ROMFS/px4fmu_common/mixers/coax.main.mix

 Coaxial helicopter mixer
-- Two servomotors act on the swashplate (90° angle on the swashplate, decoupled effect on roll and pitch).
+- Two servomotors act on the swashplate (90 degree angle on the swashplate, decoupled effect on roll and pitch).
 - No collective pitch.
 - One motor per rotor.
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