Now that the arch_{spin,read,write}_relax macros default to cpu_relax(),
remove the redundant definitions for parisc.

Cc: Helge Deller <deller@gmx.de>
Signed-off-by: NWill Deacon <will.deacon@arm.com>
Signed-off-by: NHelge Deller <deller@gmx.de>