apparmor: remove useless static inline function is_deleted

The inlined function is_deleted is redundant, it is not called at all
from any function in security/apparmor/file.c and so it can be removed.

Cleans up clang scan build warning:
security/apparmor/file.c:153:20: warning: unused function
'is_deleted' [-Wunused-function]

Signed-off-by: Colin Ian King <colin.i.king@gmail.com>
Signed-off-by: John Johansen <john.johansen@canonical.com>