maint: Remove unused '--with-nix-prefix' configure option.

* configure.ac: Remove '--with-nix-prefix' and 'NIX_PREFIX' variable.
* Makefile.am (AM_DISTCHECK_CONFIGURE_FLAGS): Remove '--with-nix-prefix'.

Makefile.am

@@ -722,7 +722,6 @@ AM_DISTCHECK_CONFIGURE_FLAGS =			\
   --localstatedir="$$dc_install_base/var"	\
   --with-libgcrypt-prefix="$(LIBGCRYPT_PREFIX)"	\
   --with-libgcrypt-libdir="$(LIBGCRYPT_LIBDIR)"	\
-  --with-nix-prefix="$(NIX_PREFIX)"		\
   --enable-daemon				\
   ac_cv_guix_test_root="$(GUIX_TEST_ROOT)"
 

configure.ac

@@ -194,19 +194,6 @@ AC_SUBST([GZIP])
 AC_SUBST([BZIP2])
 AC_SUBST([XZ])
 
-AC_ARG_WITH([nix-prefix],
-  [AS_HELP_STRING([--with-nix-prefix=DIR],
-    [search for Nix in DIR (for testing purposes and '--disable-daemon' builds)])],
-  [case "$withval" in
-    yes|no) ;;
-    *)
-      NIX_PREFIX="$withval"
-      PATH="$NIX_PREFIX/bin:$PATH"; export PATH
-      AC_SUBST([NIX_PREFIX])
-      ;;
-   esac],
-  [])
-
 AC_ARG_WITH([nixpkgs],
   [AS_HELP_STRING([--with-nixpkgs=DIR],
     [search for Nixpkgs in DIR (for testing purposes only)])],