rtas_service_present() has no more users.

rtas_function_implemented() is now the appropriate API for determining
whether a given RTAS function is available to call.
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-v1-4-61847655c51f@linux.ibm.com