Refactor handling of RC input data

- Remove globals rcData and rcRaw
- Use accesor functions to retrieve RX input data
- Refactor rx.c to use an array of structs instead of multiple arrays
- Drop the RC Preview menu from CMS. Implementing this without
globals requires significant flash and the usefulness of this menu
is questionable. If we get complains, we can add it back later.

Flash usage goes down ~250 bytes due to the removed menu. Final
binary is mostly unaffected, since LTO is able to inline the new
accessor functions in most cases.

src/main/fc/rc_modes.c

@@ -102,7 +102,7 @@ bool isRangeActive(uint8_t auxChannelIndex, const channelRange_t *range)
     // No need to constrain() here, since we're testing for a closed range defined
     // by the channelRange_t. If channelValue has an invalid value, the test will
     // be false anyway.
-    uint16_t channelValue = rcData[auxChannelIndex + NON_AUX_CHANNEL_COUNT];
+    uint16_t channelValue = rxGetChannelValue(auxChannelIndex + NON_AUX_CHANNEL_COUNT);
     return (channelValue >= CHANNEL_RANGE_MIN + (range->startStep * CHANNEL_RANGE_STEP_WIDTH) &&
             channelValue < CHANNEL_RANGE_MIN + (range->endStep * CHANNEL_RANGE_STEP_WIDTH));
 }