btrfs: remove BTRFS_REF_LAST from enum btrfs_ref_type

Commit b28b1f0c ("btrfs: delayed-ref: Introduce better documented
delayed ref structures") introduced BTRFS_REF_LAST, which can be used
for sanity checking, e.g. in switch/case or for loops.

In btrfs_ref_type() there is an assertion

  ASSERT(ref->type == BTRFS_REF_DATA || ref->type == BTRFS_REF_METADATA);

to validate the values so we don't need the ending enum.

Signed-off-by: Yangtao Li <frank.li@vivo.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>