powerpc/atomics: Remove unused function

Remove arch_atomic_try_cmpxchg_lock function as it is no longer used
since commit 9f61521c ("powerpc/qspinlock: powerpc qspinlock
implementation")

Signed-off-by: Nysal Jan K.A <nysal@linux.ibm.com>
Reviewed-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/20230224103940.1328725-1-nysal@linux.ibm.com