build: Remove 'gnu/packages/bootstrap' and its binaries.

* gnu/local.mk (bootstrapdir, bootstrap_i686_linuxdir)
(bootstrap_armhf_linuxdir, bootstrap_aarch64_linuxdir)
(bootstrap_mips64el_linuxdir, dist_bootstrap_i686_linux_DATA)
(dist_bootstrap_armhf_linux_DATA, dist_bootstrap_aarch64_linux_DATA)
(dist_bootstrap_mips64el_linux_DATA): Remove.
(set-bootstrap-executable-permissions): Remove target.
* Makefile.am (install-data-hook): Remove dependency on
'set-bootstrap-executable-permissions'.
* gnu/packages/bootstrap: Remove directory.
* tests/search-paths.scm ("evaluate-search-paths, separator is #f"):
Adjust to match .../aux-files/linux-libre.