Drop PCA9685 support

[DzikuVx]

No description provided.

[stronnag]

Sigh. Damn it, dman it breaks blackbox_decode for all previous firmware version by moving up the enums rc/runtime_config.h rather than just obsoleting the removed item.

Mea culpa, it doesn't.

[Windoofs]

Is there a technical reason why the support was removed? I am shure @DzikuVx has a bulletproof argument for this decision. Maybe the anonymous telemetry collected via Configurator showed the lack of usage in the real word ?

For my usage I was able to revert the commits in 5.0.0-RC3 and build the needed targets. Testing on hardware will be done on the weekend. The PCA9685 was a welcomed "hack" to utilize cheaper/older MCUs for bigger/smarter drones. This is handy feature during a global chip crisis.

This was the only issue while updating my fleet to 5.0.0-RC3.

[DzikuVx]

@Windoofs yes, almost nobody was using it. A few cases in total. The suggested way of handling it, it to output servos as SBUS stream and use SBUS-to-PWM decoder when needed. That's faster and more reliable than I2C connected PCA9685