object_type: remove unused object type flags

The two "reserved" bits in `git_object_t` are unused.  They were
included for completeness, but downstream users should never use them
and they should not have been made public.

These values are never set.

With the refactoring of `git_otype` into `git_object_t`, we can remove
these from the new API.  They will remain in the old (deprecated) API
in the unlikely event that someone was using them.

include/git2/types.h

@@ -70,12 +70,10 @@ typedef int64_t git_time_t;
 typedef enum {
 	GIT_OBJECT_ANY =      -2, /**< Object can be any of the following */
 	GIT_OBJECT_BAD =      -1, /**< Object is invalid. */
-	GIT_OBJECT__EXT1 =     0, /**< Reserved. */
 	GIT_OBJECT_COMMIT =    1, /**< A commit object. */
 	GIT_OBJECT_TREE =      2, /**< A tree (directory listing) object. */
 	GIT_OBJECT_BLOB =      3, /**< A file revision object. */
 	GIT_OBJECT_TAG =       4, /**< An annotated tag object. */
-	GIT_OBJECT__EXT2 =     5, /**< Reserved. */
 	GIT_OBJECT_OFS_DELTA = 6, /**< A delta, base is given by an offset. */
 	GIT_OBJECT_REF_DELTA = 7, /**< A delta, base is given by object id. */
 } git_object_t;