PM / hibernate: Remove unused function

Remove the function get_safe_write_buffer() that is not used anywhere.

This was partially found by using a static code analysis program called cppcheck.
Signed-off-by: Rickard Strandqvist <rickard_strandqvist@spectrumdigital.se>
Acked-by: Pavel Machek <pavel@ucw.cz>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>

kernel/power/snapshot.c

@@ -2310,8 +2310,6 @@ static inline void free_highmem_data(void)
 		free_image_page(buffer, PG_UNSAFE_CLEAR);
 }
 #else
-static inline int get_safe_write_buffer(void) { return 0; }
-
 static unsigned int
 count_highmem_image_pages(struct memory_bitmap *bm) { return 0; }