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

arch/powerpc/include/asm/rtas.h

@@ -542,8 +542,6 @@ static inline void pSeries_coalesce_init(void) { }
 static inline void rtas_initialize(void) { }
 #endif
 
-extern int call_rtas(const char *, int, int, unsigned long *, ...);
-
 #ifdef CONFIG_HV_PERF_CTRS
 void read_24x7_sys_info(void);
 #else