powerpc: Remove UDBG_RTAS_CONSOLE

The IBM Cell blade support was the last user of UDBG_RTAS_CONSOLE.

Although it's still possible to build it via
PPC_EARLY_DEBUG_UDBG_RTAS_CONSOLE, AFAIK it's not useful on any
other platfoms, because only Cell and JS20 era machines provided the
RTAS get/put-term-char functions.

If anyone is using it or needs it we can always resurrect it from git.

Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Signed-off-by: Madhavan Srinivasan <maddy@linux.ibm.com>
Link: https://patch.msgid.link/20241218105523.416573-19-mpe@ellerman.id.au