Glossary: remove items with no description if doced elsewhere

For example, remove stuff that is tied to a type (per discussion on #622)

doc/Language/glossary.pod6

@@ -242,9 +242,6 @@ C<True> or C<False>.
 =head1 bytecode
 X<|bytecode>
 
-=head1 Callable
-X<|Callable>
-
 =head1 Camelia
 X<|Camelia>
 
@@ -328,12 +325,6 @@ L<multi-dispatch|#Multi-Dispatch> by using a different constraint:
     say abbreviate("Worthington");   # Worthin...
     say abbreviate("Mäsak");         # Mäsak
 
-=head1 Container
-X<|Container>
-
-=head1 Cool
-X<|Cool>
-
 =head1 dalek
 X<|dalek>
 
@@ -654,9 +645,6 @@ also L<https://en.wikipedia.org/wiki/Larry_Wall>.
 =head1 Lexing
 X<|Lexing>
 
-=head1 List
-X<|List>
-
 =head1 Literal
 X<|Literal>
 
@@ -738,14 +726,6 @@ MoarVM has some similarities with the Hotspot VM so you may peruse its
 L<glossary|http://openjdk.java.net/groups/hotspot/docs/HotSpotGlossary.html>
 for entries missing from the present one.
 
-=head1 Module
-X<|Module>
-
-=head1 Mu
-X<|Mu>
-
-The mystical class from which all other classes are derived (無).
-
 =head1 Multi-Dispatch
 X<|Multi-Dispatch>
 
@@ -820,9 +800,6 @@ See L<Wikipedia article|https://en.wikipedia.org/wiki/Operating_system>
 =head1 Parse Tree
 X<|Parse Tree>
 
-=head1 Package
-X<|Package>
-
 =head1 Parameter
 X<|Parameter>
 
@@ -937,9 +914,6 @@ X<|rvalue>
 A value that can be used on the right hand side of an assignment.  See also
 L<#lvalue>.
 
-=head1 Scalar
-X<|Scalar>
-
 =head1 Sigil
 X<|Sigil>