parisc: Kconfig: remove ARCH_DISCARD_MEMBLOCK

Since commit 350e88ba ("mm: memblock: make keeping memblock memory
opt-in rather than opt-out") the default behaviour is to discard memblock
data after init and the ARCH_DISCARD_MEMBLOCK is obsolete.

Remove it.

Signed-off-by: Mike Rapoport <rppt@linux.ibm.com>
Signed-off-by: Helge Deller <deller@gmx.de>