Don't use anymore async-bytecomp-package-mode

There is now packages installing other packages asynchronously (even
helm now has its package manager async), so this mode is clashing with
such packages.  Also heavy modifications in helm-core are less
frequent nowaday (macro renaming etc..).

helm-core.el

@@ -34,12 +34,6 @@
 (require 'helm-multi-match)
 (require 'helm-source)
 
-;; Ensure async-bytecomp is used even with helm-core package.
-(declare-function async-bytecomp-package-mode "ext:async-bytecomp.el")
-(when (require 'async-bytecomp nil t)
-  (and (fboundp 'async-bytecomp-package-mode)
-       (async-bytecomp-package-mode 1)))
-
 ;; Setup completion styles for helm-mode
 (helm--setup-completion-styles-alist)