UML: Clean up asm/system.h

Remove duplicates and unused prototypes.
Signed-off-by: Jan Kiszka <jan.kiszka@web.de>
Acked-by: WANG Cong <xiyou.wangcong@gmail.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>

arch/um/include/asm/system.h

@@ -3,11 +3,8 @@
 
 #include "sysdep/system.h"
 
-extern void *switch_to(void *prev, void *next, void *last);
-
 extern int get_signals(void);
 extern int set_signals(int enable);
-extern int get_signals(void);
 extern void block_signals(void);
 extern void unblock_signals(void);