powerpc: Remove obsoleted declaration for _get_SP

The implementation of _get_SP() was removed in commit f4db1967
("[POWERPC] Remove  _get_SP"), remove the now obsolete declaration.
Signed-off-by: Gaosheng Cui <cuigaosheng1@huawei.com>
Reviewed-by: Christophe Leroy <christophe.leroy@csgroup.eu>
[mpe: Update change log to refer to correct commit per Christophe]
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/20240822130609.786431-2-cuigaosheng1@huawei.com

arch/powerpc/kernel/process.c

@@ -72,8 +72,6 @@
 #define TM_DEBUG(x...) do { } while(0)
 #endif
 
-extern unsigned long _get_SP(void);
-
 #ifdef CONFIG_PPC_TRANSACTIONAL_MEM
 /*
  * Are we running in "Suspend disabled" mode? If so we have to block any