MIPS: dec: prom: Remove unused unregister_prom_console() declaration

The unregister_prom_console() has been removed since
commit 36a88530 ("[MIPS] Fix and cleanup the mess that a dozen
prom_printf variants are."), so remove it.
Signed-off-by: Gaosheng Cui <cuigaosheng1@huawei.com>
Acked-by: Maciej W. Rozycki <macro@orcam.me.uk>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>

arch/mips/include/asm/dec/prom.h

@@ -160,6 +160,5 @@ extern void prom_identify_arch(u32);
 extern void prom_init_cmdline(s32, s32 *, u32);
 
 extern void register_prom_console(void);
-extern void unregister_prom_console(void);
 
 #endif /* _ASM_DEC_PROM_H */