powerpc/rtas: Drop declaration of undefined call_rtas() function

The call_rtas() function has never been a part of arch/powerpc, and
its implementation was removed from arch/ppc by 0a26b136 ("ppc:
Remove CHRP, POWER3 and POWER4 support from arch/ppc").

Signed-off-by: Nathan Lynch <nathanl@linux.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/20231106-rtas-trivial-v1-3-61847655c51f@linux.ibm.com