btrfs: remove redundant initialization of variable dirty in btrfs_update_time()

The variable dirty is initialized with a value that is never read, it
is being re-assigned later on. Remove the redundant initialization.
Cleans up clang scan build warning:

  fs/btrfs/inode.c:5965:7: warning: Value stored to 'dirty' during its
  initialization is never read [deadcode.DeadStores]

Signed-off-by: Colin Ian King <colin.i.king@gmail.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>