Use MELPA for perl6-mode package

The mentioned issue was resolved:
https://github.com/melpa/melpa/issues/5261

CHANGELOG.develop

@@ -279,7 +279,7 @@ Benner and Paweł Siudak):
 - kotlin (thanks to Shanavas M)
 - pact (thanks to Colin Woodbury)
 - perl5 (thanks to Troy Hinckley, Jinseop Kim and Michael Rohleder)
-- perl6 (thanks to Bahtiar Gadimov)
+- perl6 (thanks to Bahtiar Gadimov and yuhan0)
 - protobuf (thanks to Amol Mandhane)
 - restructuredtext (thanks to Wei-Wei Guo and Kalle Lindqvist)
 - semantic-web (thanks to Andreas Textor)

layers/+lang/perl6/packages.el

@@ -15,10 +15,7 @@
     evil
     flycheck
     (flycheck-perl6 :requires flycheck)
-    ;; Not available in MELPA for now
-    ;; TODO check progress on issue: https://github.com/melpa/melpa/issues/5261
-    (perl6-mode :location (recipe :fetcher github
-                                  :repo "perl6/perl6-mode"))
+    perl6-mode
     ))
 
 (defun perl6/post-init-company ()