Update extension.rdoc for Ruby 3 keyword argument separation [ci skip]

This removes the discussion of behavior removed in Ruby 3.

doc/extension.rdoc

@@ -457,8 +457,6 @@ To specify whether keyword arguments are passed when calling super:
 
 RB_NO_KEYWORDS :: Do not pass keywords
 RB_PASS_KEYWORDS :: Pass keywords, final argument should be a hash of keywords
-RB_PASS_EMPTY_KEYWORDS :: Pass empty keywords (not included in arguments)
-                          (this will be removed in Ruby 3.0)
 RB_PASS_CALLED_KEYWORDS :: Pass keywords if current method was called with
                            keywords, useful for argument delegation
 
@@ -1423,28 +1421,6 @@ rb_scan_args(int argc, VALUE *argv, const char *fmt, ...) ::
                                             ; argument captured as a hash.
                                             ; If keyword arguments are not
                                             ; provided, returns nil.
-                                            ;
-                                            ; Currently, will also consider
-                                            ; final argument as keywords if
-                                            ; it is a hash or can be
-                                            ; converted to a hash with
-                                            ; #to_hash.  When the last
-                                            ; argument is nil, it is
-                                            ; captured if it is not
-                                            ; ambiguous to take it as
-                                            ; empty option hash; i.e. '*'
-                                            ; is not specified and
-                                            ; arguments are given more
-                                            ; than sufficient.
-                                            ;
-                                            ; However, handling final
-                                            ; argument as keywords if
-                                            ; method was not called with
-                                            ; keywords (whether final
-                                            ; argument is hash or nil) is
-                                            ; deprecated. In that case, a
-                                            ; warning will be emitted, and
-                                            ; in Ruby 3.0 it will be an error.
     sym-for-block-arg              := "&"   ; Indicates that an iterator
                                             ; block should be captured if
                                             ; given
@@ -1469,8 +1445,6 @@ rb_scan_args_kw(int kw_splat, int argc, VALUE *argv, const char *fmt, ...) ::
   RB_SCAN_ARGS_PASS_CALLED_KEYWORDS :: Same behavior as +rb_scan_args+.
   RB_SCAN_ARGS_KEYWORDS :: The final argument should be a hash treated as
                            keywords.
-  RB_SCAN_ARGS_EMPTY_KEYWORDS :: Don't treat a final hash as keywords.
-                                 (this will be removed in Ruby 3.0)
   RB_SCAN_ARGS_LAST_HASH_KEYWORDS :: Treat a final argument as keywords if it
                                      is a hash, and not as keywords otherwise.