Eliminates one duplicate

Which goes towards #1912 but it useless anyway because #1894

Author: JJ

doc/Language/classtut.pod6

@@ -462,7 +462,7 @@ on the various other dependencies in order, giving the output:
 
 =head1 Inheritance
 
-X<|is (inheritance)>
+X<|classes, inheritance>
 
 Object Oriented Programming provides the concept of inheritance as one of
 the mechanisms for code reuse.  Perl 6 supports the ability for one

doc/Language/typesystem.pod6

@@ -378,13 +378,13 @@ C<HyperWhatever>.
     }
     E.new.em1;
 
-=head3 trait C<is>
+=head3  X<trait C<is>|classes,is>
 
 Defined as:
 
     multi sub trait_mod:<is>(Mu:U $child, Mu:U $parent)
 
-The L<trait|/type/Sub#Traits> X<C<is>|is (inheritance)> accepts a type object to be
+The L<trait|/type/Sub#Traits> C<is> accepts a type object to be
 added as a parent class of a class in its definition. To allow multiple
 inheritance the trait can be applied more than once. Adding parents to a class
 will import their methods into the target class. If the same method name occurs