Remove the 'vm-image' command that has been superseded by the 'image'
command.
* gnu/system/vm.scm (system-qemu-image): Remove it.
* guix/scripts/system.scm (system-derivation-for-action): Mark 'vm-image'
command as deprecated and use the image API to produce the VM image.
(perform-action, show-help): Adapt accordingly.
* tests/guix-system.sh: Ditto.
* doc/guix.texi (Invoking guix system,
Running Guix in a VM): Ditto.
* etc/completion/fish/guix.fish: Ditto.
* etc/completion/zsh/_guix: Ditto.
This is a followup to bc69ea2d605810cc32e13ed03d5848b8dc358b61.
* guix/scripts/build.scm (show-build-options-help): Rename
"--no-build-hook" to "--no-offload".
(%standard-build-options): Likewise, and warn when "--no-build-hook" is
passed.
* nix/nix-daemon/guix-daemon.cc (options): Add "--no-offload" and mark
"--no-build-hook" as hidden.
* guix/scripts/offload.scm: Adjust comment.
* doc/guix.texi (Invoking guix-daemon, Common Build Options): Replace
"--no-build-hook" with "--no-offload".
* etc/completion/fish/guix.fish, etc/completion/zsh/_guix: Adjust
accordingly.
