build-system/{cmake,perl}: Don't use (guix build gnu-build-system).

Previously references to '%standard-phases' in builder-side code would
refer to the wrong one.

* guix/build-system/cmake.scm (cmake-build): Remove (guix build
  gnu-build-system) from the default MODULES value.
* guix/build-system/perl.scm (perl-build): Likewise.

guix/build-system/cmake.scm

@@ -65,7 +65,6 @@
                                          (guix build gnu-build-system)
                                          (guix build utils)))
                      (modules '((guix build cmake-build-system)
-                                (guix build gnu-build-system)
                                 (guix build utils))))
   "Build SOURCE using CMAKE, and with INPUTS. This assumes that SOURCE
 provides a 'CMakeLists.txt' file as its build system."

guix/build-system/perl.scm

@@ -57,7 +57,6 @@
                                          (guix build gnu-build-system)
                                          (guix build utils)))
                      (modules '((guix build perl-build-system)
-                                (guix build gnu-build-system)
                                 (guix build utils))))
   "Build SOURCE using PERL, and with INPUTS.  This assumes that SOURCE
 provides a `Makefile.PL' file as its build system."