powerpc/xmon: Remove SPU debug and disassembly

Now that the IBM Cell Blade support is removed, the xmon SPU support is
effectively unusable. That is because PS3 doesn't implement udbg_getc
which is required to send input to xmon.

So remove the xmon SPU support.

Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Signed-off-by: Madhavan Srinivasan <maddy@linux.ibm.com>
Link: https://patch.msgid.link/20241218105523.416573-6-mpe@ellerman.id.au