futex: Remove unused empty compat_exit_robust_list()

Commit ba31c1a485 ("futex: Move futex exit handling into futex code")
introduced compat_exit_robust_list() with a full-fledged implementation for
CONFIG_COMPAT, and an empty-body function for !CONFIG_COMPAT.

However, compat_exit_robust_list() is only used in futex_mm_release() under
#ifdef CONFIG_COMPAT.

Hence for !CONFIG_COMPAT, make CC=clang W=1 warns:

  kernel/futex.c:314:20:
    warning: unused function 'compat_exit_robust_list' [-Wunused-function]

There is no need to declare the unused empty function for !CONFIG_COMPAT.

Simply remove it.

Signed-off-by: Lukas Bulwahn <lukas.bulwahn@gmail.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Nathan Chancellor <natechancellor@gmail.com>
Link: https://lore.kernel.org/r/20201113172012.27221-1-lukas.bulwahn@gmail.com

kernel/futex.c

@@ -310,8 +310,6 @@ static inline bool should_fail_futex(bool fshared)
 
 #ifdef CONFIG_COMPAT
 static void compat_exit_robust_list(struct task_struct *curr);
-#else
-static inline void compat_exit_robust_list(struct task_struct *curr) { }
 #endif
 
 /*