Replace external commands with fish builtins - #12911

Get's rid of all leftover occurrences of 'cut' and one occurence of 'tr' in the fish completions.

Fixes part of issue #6520

List of all open fixes from that issue under their category. I've added the commit for files where the command has already been replaced and I could find out where. I've written 'already done?' where there was no occurrence and I couldn't easily find out where it was replaced. All other entries in this list have been fixed by me.

cut

• completions/fossil.fish
• completions/machinectl.fish
• completions/modprobe.fish
• completions/pactree.fish already done?
• completions/pkgfile.fish already done?
• completions/xprop.fish f66285d
• functions/__fish_complete_groups.fish f66285d
• functions/__fish_complete_gpg_user_id.fish f66285d
• functions/__fish_crux_packages.fish f66285d
• functions/__fish_is_zfs_feature_enabled.fish 21ddfab
• functions/__fish_print_packages.fish already done?
• functions/__fish_systemctl_services.fish f66285d
• functions/fish_svn_prompt.fish

tr

• completions/aura.fish
• completions/grunt.fish already done?
• completions/pactree.fish already done?
• completions/pkgfile.fish already done?
• completions/rfkill.fish already done?
• completions/rsync.fish already done?
• functions/__fish_make_completion_signals.fish
• functions/__fish_parse_configure.fish file gone?
• functions/__fish_print_packages.fish already done?
• functions/__fish_print_xdg_desktop_file_ids.fish file gone?
• functions/fish_svn_prompt.fish

TODOs:

• If addressing an issue, a commit message mentions Fixes issue #<issue-number>
• Changes to fish usage are reflected in user documentation/manpages.
• Tests have been added for regressions fixed
• User-visible changes noted in CHANGELOG.rst