Load completions at completion rather than load time

What happens when we install a new plugin or remove
an existing plugin? If we don't reload completions,
Fish won't be able to tab-complete for installed or
removed plugins, e.g., continue tab-completing when
they are no longer installed.

By loading `update` and `remove` completions at
completion load time we now invoke `fisher list`
every time the user tabs after `fisher update`
or `fisher remove`.

completions/fisher.fish

@@ -1,11 +1,7 @@
-complete -c fisher -e
 complete -c fisher -x -l help -d "print usage help"
 complete -c fisher -x -l version -d "print fisher version"
 complete -c fisher -x -n "__fish_use_subcommand" -a install -d "install plugins"
 complete -c fisher -x -n "__fish_use_subcommand" -a update -d "update installed plugins"
 complete -c fisher -x -n "__fish_use_subcommand" -a remove -d "remove installed plugins"
 complete -c fisher -x -n "__fish_use_subcommand" -a list -d "list installed plugins matching <regex>"
-
-for plugin in (fisher list)
-    complete -c fisher -x -n "__fish_seen_subcommand_from update remove" -a $plugin
-end
+complete -c fisher -x -n "__fish_seen_subcommand_from update remove" -a "(fisher list)"

fisher.fish

@@ -144,8 +144,6 @@ function fisher -a cmd -d "fish plugin manager"
 
             functions -q fish_prompt || source $__fish_data_dir/functions/fish_prompt.fish
 
-            source $fisher_path/completions/fisher.fish 2>/dev/null
-
             _fisher_list >$fish_plugins
             test -s $fish_plugins || command rm -f $fish_plugins