Copter: remove gyro cal prearm check

This is no longer necessary - it has been replaced by an arming check.

If left in, this piece of code is going to be a user experience nightmare. The only solution to this check failing is to re-power the copter or to disable the INS pre-arm check. Users that don't have a GCS for feedback could wait upwards of 10-15 minutes for prearm checks to pass, only to get frustrated and re-power (the only solution). Users that do have a GCS could still wait a long time for the rest of the prearm checks to pass, only to discover that they must re-power the copter.

The result will be a lot of users disabling the check, and the result of that will be crashes.