move save_trim() from RC_Channels_Copter to Copter class

[codemaster1104]

save_trim() operates on Copter member variables (flightmode,
attitude_control, g2.rc_channels) and is more naturally a method
of the Copter class than of RC_Channels_Copter.
Changes:

• Remove save_trim() declaration and definition from RC_Channels_Copter
• Add save_trim() declaration to Copter class (under radio.cpp section)
• Implement save_trim() in ArduCopter/radio.cpp
• Update both call sites in RC_Channel_Copter.cpp to use copter.save_trim()

this is work on #32279