Eliminates duplicate reference for 'is defined'

And along with it, probably the "Trait" category of the search
part. Closes #1514

Author: JJ

doc/Type/Attribute.pod6

@@ -32,7 +32,7 @@ The usual way to obtain an object of type C<Attribute> is by introspection:
 
 =head1 Traits
 
-=head2 X<Trait is default|trait,is default (Attribute)>
+=head2 X<Trait is default|is default (Attribute)>
 
 An attribute that is assigned L<Nil|/type/Nil> will revert to its default value
 set with the trait C<is default>.

doc/Type/Mu.pod6

@@ -16,7 +16,7 @@ L<Any|/type/Any>.
 
 =head1 Methods
 
-=head2 method defined
+=head2 method defined X<|method,defined>
 
 Declared as
 

doc/Type/Routine.pod6

@@ -17,7 +17,7 @@ Routine is also the level at which
 L<multiness|/language/glossary#Multi-Dispatch> (multi subs and multi methods)
 are handled.
 
-=head2 X<Trait is default|trait,is default (Routine)>
+=head2 X<Trait is default|is default (Routine)>
 
 There is a special trait for C<Routine>s called C<is default>. This trait is
 designed as a way to disambiguate C<multi> calls that would normally

doc/Type/Variable.pod6

@@ -21,7 +21,7 @@ introspect and manipulate variables.
 
 Returns the name of the variable, including the sigil.
 
-=head2 X<trait is default|trait,is default (Variable)>
+=head2 X<trait is default|is default (Variable)>
 
     multi sub trait_mod:<is>(Variable:D, :$default!)