Remove :T smiley from specs

The :U smiley has taken the place of :T over the years, and the only
difference specified between them (where :U accepts Failures but :T does
not) doesn't appear to make sense anymore. Now :U is spec'd to mean type
objects, and :D means instances.

The only potentially significant change for rakudo is the change from :T
to :U as the default smiley for method new's invocant, but depending on
existing implementation may already be how it works.

S06-routines.pod

@@ -941,9 +941,9 @@ using the type name in from of the parameter:
 If no explicit type constraint is given, it defaults to the type of the
 surrounding package for method invocants, and to C<Any> everywhere else.
 
-A bare C<:D>, C<:U>, C<:T> or C<:_> instead of a type constraint limits the
-default type to definite, undefined, type objects or any object. The default
-still applies, so in
+A bare C<:D>, C<:U> or C<:_> instead of a type constraint limits the default
+type to definite objects (aka instances), undefined objects (aka type objects),
+or any object, respectively. The default still applies, so in
 
     class Con {
         method man(:U: :D $x)

S12-objects.pod

@@ -1620,21 +1620,19 @@ from it by appending an appropriate adverbial to its name:
 
     Int:_       Allow either defined or undefined Int values
     Int:D       Allow only defined (concrete) Int values
-    Int:U       Allow only undefined (abstract or failure) Int values
-    Int:T       Allow Int only as a type object
+    Int:U       Allow only undefined (abstract) Int values
 
 That is, these mean something like:
 
     Int:D       Int:_ where DEFINITE($_)
     Int:U       Int:_ where not(DEFINITE($_))
-    Int:T       Int:U where none(Failure)
 
 where C<DEFINITE> is a boolean macro that says whether the object
 in question has a valid concrete representation (see L</Introspection> below).
 
 In standard Perl 6, C<Int> is generally assumed to mean C<Int:_>, except
 for invocants, where the default is C<Int:D>.  (The default C<new> method
-has a prototype whose invocant is C<:T> instead, so all new methods all
+has a prototype whose invocant is C<:U> instead, so all new methods all
 default to allowing type objects.)
 
 These defaults may be changed within a lexical scope by various pragmas.