powerpc/udbg: Remove unused udbg_read()

The last user of udbg_read() was removed in 2005, in commit fca5dcd4
"Simplify and clean up the xmon terminal I/O".

Given we haven't needed it for 7 years we can probably drop it.

Signed-off-by: Michael Ellerman <michael@ellerman.id.au>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>