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/fc_msp.c

@@ -507,7 +507,7 @@ static bool mspFcProcessOutCommand(uint16_t cmdMSP, sbuf_t *dst, mspPostProcessF
 
     case MSP_RC:
         for (int i = 0; i < rxRuntimeConfig.channelCount; i++) {
-            sbufWriteU16(dst, rcRaw[i]);
+            sbufWriteU16(dst, rxGetRawChannelValue(i));
         }
         break;