Remove 'guix-register' and its traces.

Remove 'guix-register' and its traces.

* Makefile.am (SH_TESTS): Remove tests/guix-register.sh.
* build-aux/pre-inst-env.in (GUIX_REGISTER): Remove.
* gnu/build/install.scm (directives): Remove outdated comment.
* gnu/build/vm.scm (root-partition-initializer): Update comment.
* gnu/packages/package-management.scm (guix-register): Remove.
* guix/config.scm.in (%sbindir, %guix-register-program): Remove.
* guix/scripts/system.scm (install): Adjust docstring.
* guix/self.scm (make-config.scm): Remove #:guix.  Do not generate
%sbindir and %guix-register-program.
(specification->package): Remove "guix".
* nix/guix-register/guix-register.cc: Remove.
* nix/libstore/store-api.cc (decodeValidPathInfo): Remove.
* nix/libstore/store-api.hh (decodeValidPathInfo): Remove declaration.
* nix/local.mk (sbin_PROGRAMS, guix_register_SOURCES)
(guix_register_CPPFLAGS, guix_register_LDFLAGS): Remove.
* tests/guix-register.sh: Remove.