btrfs: remove unnecessary prototype declarations at disk-io.c

[ Upstream commit 184533e3

 ]

We have a few static functions at disk-io.c for which we have a forward
declaration of their prototype, but it's not needed because all those
functions are defined before they are called, so remove them.

Signed-off-by: Filipe Manana <fdmanana@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>
Stable-dep-of: fb33eb2e

 ("btrfs: fix leak of qgroup extent records after transaction abort")
Signed-off-by: Sasha Levin <sashal@kernel.org>