btrfs: add and use helper to remove extent map from its inode's tree

Move the common code to remove an extent map from its inode's tree into a
helper function and use it, reducing duplicated code.

Reviewed-by: Josef Bacik <josef@toxicpanda.com>
Signed-off-by: Filipe Manana <fdmanana@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>