sys/vfs/hammer: remove obsolete comments on btree, etc

sys/vfs/hammer: remove obsolete comments on btree, etc

- hunk1: Use 'type' instead of 'subtype'. subtype was a different
  idea from type originally which no longer exist.

- hunk2: There is no longer a distiction of normal/meta btree node.
  It's either internal or leaf node, plus record elements within leaf.

- hunk3: Remove comments on subtype field that no longer exist,
  and add a comment on node size with 63-way btree.

- hunk4: These macros were only used in very early stage of hammer
  development.

- hunk5-6: Minor cleanups.