emacs: make sure the pinned package-build is used

package-build is also in MELPA.  Currently, two packages depend on
it.  When building such a package, package-build is added to
EMACSLOADPATH by the setup hook.  Previously, package-initialize is
called in the buildPhase of melpaBuild, which may cause package-build
from MELPA to be used instead of the pinned one depending on the
entries of EMACSLOADPATH.

This patch removes the unneeded package-initialize to make sure the
pinned package-build is used.

Accidentally, this patch reduces build time of emacsPackages from 30
minutes to 24 minutes on my machine.

pkgs/applications/editors/emacs/build-support/melpa2nix.el

@@ -1,6 +1,3 @@
-(require 'package)
-(package-initialize)
-
 (require 'package-recipe)
 (require 'package-build)