object: deprecate git_object__size for removal

In #5118 we remove the double-underscore to make it a normally-named public
function. However, this is not an interesting function outside of the library
and it takes up a name for something that could be more useful.

Remove the single-underscore version as we have not done any releases with it.

include/git2/object.h

@@ -185,20 +185,6 @@ GIT_EXTERN(git_object_t) git_object_string2type(const char *str);
  */
 GIT_EXTERN(int) git_object_typeisloose(git_object_t type);
 
-/**
- * Get the size in bytes for the structure which
- * acts as an in-memory representation of any given
- * object type.
- *
- * For all the core types, this would the equivalent
- * of calling `sizeof(git_commit)` if the core types
- * were not opaque on the external API.
- *
- * @param type object type to get its size
- * @return size in bytes of the object
- */
-GIT_EXTERN(size_t) git_object_size(git_object_t type);
-
 /**
  * Recursively peel an object until an object of the specified type is met.
  *