Update hellbender_2350A_devboard.h for release version of the board.

[CBJamo]

We've only used the old version of this board internally so there's no reason to maintain both versions.

Fixes #2440

[lurch]

Can you rebase this onto the latest version of develop please? Note that (after #2397) // pico_cmake_set PICO_PLATFORM=rp2350 has now become pico_board_cmake_set(PICO_PLATFORM, rp2350) https://github.com/raspberrypi/pico-sdk/blob/develop/src/boards/include/boards/hellbender_2350A_devboard.h#L15
(as well as // pico_cmake_set_default name = value changing to pico_board_cmake_set_default(name, value)).

EDIT: In #2442 I've updated tools/check_board_header.py to display better error messages.

[lurch]

...and if I make those changes locally, tools/check_board_header.py complains with:
Exception: src/boards/include/boards/hellbender_2350A_devboard.h:81 PICO_DEFAULT_SPI_CSN_PIN is set to 28 which isn't a valid pin for CSN on SPI 0
and Table 3 in https://datasheets.raspberrypi.com/rp2350/rp2350-datasheet.pdf indeed shows that GPIO28 has SPI function SPI1 RX.

[CBJamo]

Rebased, sorry about missing that.

I removed the default cs pin define, the bus is connected to both an expansion header and a micro sd card.

[lurch]

Thanks for the update 👍