Remove stale lingo

Part of #4779

Author: coke

doc/Language/glossary.rakudoc

@@ -538,10 +538,6 @@ No Such Thing
 Short for "optimization", usually in either the context of
 L<spesh|#Spesh> or JIT.
 
-=head2 X<PB|Reference,PB>
-
-Short for "problem". As in "that's not the pb".
-
 =head2 X<PR|Reference,PR>
 
 See L<Pull request|#Pull request>.
@@ -550,20 +546,10 @@ See L<Pull request|#Pull request>.
 
 Perl 5
 
-=head2 X<P6|Reference,P6>
-
-Raku (née Perl 6)
-
 =head2 X<RSN|Reference,RSN>
 
 Real Soon Now.
 
-=head2 X<RT|Reference,RT>
-
-Request Tracker (L<https://rt.perl.org/>). The place where all the bugs
-related to L<Rakudo|#Rakudo> used to live. Nowadays, the Github issue tracker of
-the rakudo/rakudo repository is used for that.
-
 =head2 X<TIMTOWTDI|Reference,TIMTOWTDI>
 
 An alternative form of L<TIMTOWTDI|#TMTOWTDI>, explicitly including the "is" from
@@ -589,10 +575,6 @@ Works For Me
 
 Work In Progress
 
-=head2 X<WP|Reference,WP>
-
-Wikipedia
-
 =head2 X<WW|Reference,WW>
 
 Short for C<wrong window>. When on L<IRC|#IRC>, someone types something in