[io grant] Toss IO::Path.child-secure

We'll make .child be .child-secure around 6.d time instead.

Discussion: https://irclog.perlgeek.de/perl6-dev/2017-04-17#i_14439386

Author: zoffixznet

doc/Type/IO/Path.pod6

@@ -136,33 +136,11 @@ C<IO::Path>.
 
     method child(IO::Path:D: Str() $childname --> IO::Path:D)
 
-Alias for L«C<.add>|/routine/add». See also
-L«C<.child-secure>|/routine/child-secure»
-
-=head2 method child-secure
-
-  method child-secure(IO::Path:D: Str() $childname --> IO::Path:D)
-
-Same as L«C<.child>|/routine/child», except it will
-B«fully L<resolve|/routine/resolve>» the resultant path and check that it is in
-fact a child of the invocant. On success, returns the resultant path;
-L«fails|/routine/fail» with C<X::IO::Resolve> if the path could not be
-completely resolved or with C<X::IO::NotAChild> if the resultant path is not
-a child of the invocant.
-
-B<Note:> since the path has to be fully resolved, the method can still fail
-with C<X::IO::Resolve> even for paths that would be child paths, if some of
-the directories do not exist yet. The last part of the path does not have to
-exist for the routine to succeed.
-
-=for code :skip-test
-my $parent = 'foo'.IO.mkdir;
-say $parent.child-secure: 'bar';     # OUTPUT: «"foo/bar".IO␤»
-say $parent.child-secure: '../bar';  # Fails with X::IO::NotAChild
-say $parent.child-secure: 'bar/ber'; # Fails with X::IO::Resolve
-
-$parent.child('bar').mkdir; # create 'bar' so the call below succeeds
-say $parent.child-secure: 'bar/ber'; # OUTPUT: «"foo/bar/ber".IO␤»
+Alias for L«C<.add>|/routine/add». B<NOTE:> C<.child> will be switched to
+secure version around 6.d release time, which will C<fail()> with non-child
+paths and is also slower, since it has to do more work. Use
+L«C<.add>|/routine/add» if your code does not need to guarantee the added
+path is in fact a child path.
 
 =head2 method extension