build-system: Remove irrelevant special case.

* guix/build-system/gnu.scm (gnu-build, gnu-cross-build): Remove comment
  about case where GUILE is a derivation path.
* guix/build-system/cmake.scm (cmake-build): Remove case where GUILE is
  a derivation path.
* guix/build-system/perl.scm (perl-build): Likewise.
* guix/build-system/python.scm (python-build): Likewise.
* guix/build-system/ruby.scm (ruby-build): Likewise.
* guix/build-system/trivial.scm (guile-for-build): Likewise.

guix/build-system/cmake.scm

@@ -98,8 +98,6 @@ provides a 'CMakeLists.txt' file as its build system."
     (match guile
       ((? package?)
        (package-derivation store guile system))
-      ((and (? string?) (? derivation-path?))
-       guile)
       (#f                                         ; the default
        (let* ((distro (resolve-interface '(gnu packages commencement)))
               (guile  (module-ref distro 'guile-final)))

guix/build-system/gnu.scm

@@ -344,8 +344,6 @@ are allowed to refer to."
     (match guile
       ((? package?)
        (package-derivation store guile system))
-      ;; ((and (? string?) (? derivation-path?))
-      ;;  guile)
       (#f                                         ; the default
        (let* ((distro (resolve-interface '(gnu packages commencement)))
               (guile  (module-ref distro 'guile-final)))
@@ -530,8 +528,6 @@ platform."
     (match guile
       ((? package?)
        (package-derivation store guile system))
-      ;; ((and (? string?) (? derivation-path?))
-      ;;  guile)
       (#f                                         ; the default
        (let* ((distro (resolve-interface '(gnu packages commencement)))
               (guile  (module-ref distro 'guile-final)))

guix/build-system/perl.scm

@@ -90,8 +90,6 @@ provides a `Makefile.PL' file as its build system."
     (match guile
       ((? package?)
        (package-derivation store guile system))
-      ((and (? string?) (? derivation-path?))
-       guile)
       (#f                                         ; the default
        (let* ((distro (resolve-interface '(gnu packages commencement)))
               (guile  (module-ref distro 'guile-final)))

guix/build-system/python.scm

@@ -138,8 +138,6 @@ provides a 'setup.py' file as its build system."
     (match guile
       ((? package?)
        (package-derivation store guile system))
-      ((and (? string?) (? derivation-path?))
-       guile)
       (#f                                         ; the default
        (let* ((distro (resolve-interface '(gnu packages commencement)))
               (guile  (module-ref distro 'guile-final)))

guix/build-system/ruby.scm

@@ -77,8 +77,6 @@
     (match guile
       ((? package?)
        (package-derivation store guile system))
-      ((and (? string?) (? derivation-path?))
-       guile)
       (#f
        (let* ((distro (resolve-interface '(gnu packages commencement)))
               (guile  (module-ref distro 'guile-final)))

guix/build-system/trivial.scm

@@ -29,8 +29,6 @@
   (match guile
     ((? package?)
      (package-derivation store guile system))
-    ((and (? string?) (? derivation-path?))
-     guile)
     (#f                                         ; the default
      (let* ((distro (resolve-interface '(gnu packages commencement)))
             (guile  (module-ref distro 'guile-final)))