build: Remove code to download the Guile bootstrap tarball.

build: Remove code to download the Guile bootstrap tarball.

* build-aux/download.scm: Remove.
* Makefile.am (EXTRA_DIST): Remove it.
(check-system): Remove dependency on $(BOOTSTRAP_GUILE_TARBALLS).
* gnu/local.mk (nodist_bootstrap_x86_64_linux_DATA)
(nodist_bootstrap_i686_linux_DATA, nodist_bootstrap_armhf_linux_DATA)
(nodist_bootstrap_aarch64_linux_DATA, nodist_bootstrap_mips64el_linux_DATA)
(BOOTSTRAP_GUILE_TARBALLS, DISTCLEANFILES, DOWNLOAD_FILE)
(%D%/packages/bootstrap/x86_64-linux/guile-2.0.9.tar.xz)
(%D%/packages/bootstrap/i686-linux/guile-2.0.9.tar.xz)
(%D%/packages/bootstrap/armhf-linux/guile-2.0.11.tar.xz)
(%D%/packages/bootstrap/aarch64-linux/guile-2.0.14.tar.xz)
(%D%/packages/bootstrap/mips64el-linux/guile-2.0.9.tar.xz): Remove.
* build-aux/test-env.in: Add "guix download" invocation when
gnu/packages/bootstrap/guile* exists.