utils: 'modify-phases' no longer introduces quotes.

Suggested by Taylan Ulrich Bayırlı/Kammer <taylanbayirli@gmail.com>.

* guix/build/utils.scm (%modify-phases): Remove quotes.
* guix/build/cmake-build-system.scm (%standard-phases): Adjust
  accordingly.
* guix/build/glib-or-gtk-build-system.scm (%standard-phases): Likewise.
* guix/build/gnu-dist.scm (%dist-phases): Likewise.
* guix/build/perl-build-system.scm (%standard-phases): Likewise.
* guix/build/python-build-system.scm (%standard-phases): Likewise.
* guix/build/ruby-build-system.scm (%standard-phases): Likewise.
* guix/build/waf-build-system.scm (%standard-phases): Likewise.
* gnu/packages/bash.scm, gnu/packages/code.scm, gnu/packages/gl.scm,
  gnu/packages/gnome.scm, gnu/packages/graphics.scm,
  gnu/packages/image.scm, gnu/packages/key-mon.scm,
  gnu/packages/ocr.scm, gnu/packages/plotutils.scm,
  gnu/packages/search.scm, gnu/packages/video.scm: Likewise.