class/audio: remove unused audio20_control_request_t

After the UAC2 examples switched to tusb_control_request_t with
TU_U16_HIGH/LOW() extraction, audio20_control_request_t is no longer
referenced anywhere in the tree. It is a byte-overlay of the setup
packet whose bChannelNumber/bControlSelector/bInterface/bEntityID
sub-byte fields silently misread on big-endian once wValue/wIndex are
converted to host order (tu_le16toh in dcd.h), so leaving it in the
public header is a latent BE trap; the BE bitfield guard previously
added to its bmRequestType_bit only masked that by guarding byte 0.

Drop the struct entirely. Callers should use tusb_control_request_t and
TU_U16_LOW/HIGH(wValue|wIndex), matching audio_device.c and the examples.

Co-Authored-By: Claude Opus 4.8 (1M context) <[email protected]>

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