[ruby/English] Remove $IGNORECASE

`$=` has been obsolete and has no effect since 1.9.

https://github.com/ruby/English/commit/121939695a

lib/English.rb

@@ -39,7 +39,6 @@
 #  $PROCESS_ID::              $$
 #  $CHILD_STATUS::            $?
 #  $LAST_MATCH_INFO::         $~
-#  $IGNORECASE::              $=
 #  $ARGV::                    $*
 #  $MATCH::                   $&
 #  $PREMATCH::                $`
@@ -151,9 +150,6 @@ alias $CHILD_STATUS            $?
 # scope.
 alias $LAST_MATCH_INFO         $~
 
-# This variable is no longer effective. Deprecated.
-alias $IGNORECASE              $=
-
 # An array of strings containing the command-line
 # options from the invocation of the program. Options
 # used by the Ruby interpreter will have been