btrfs: remove unused flag EXTENT_BUFFER_IN_TREE

This flag is set after inserting the eb to the buffer tree and cleared
on it's removal.  It was added in commit 34b41ace ("Btrfs: use a
bit to track if we're in the radix tree") and wanted to make use of it,
faa2dbf0 ("Btrfs: add sanity tests for new qgroup accounting
code"). Both are 10+ years old, we can remove the flag.

Signed-off-by: Daniel Vacek <neelx@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>