Move zsh completion logic to new subcommand: rm

Signed-off-by: Steve Durrheimer <s.durrheimer@gmail.com>

contrib/completion/zsh/_docker

@@ -986,8 +986,11 @@ __docker_image_subcommand() {
                 "($help -): :__docker_complete_images" && ret=0
             ;;
         (rm)
-            words[1]='rmi'
-            __docker_subcommand && ret=0
+            _arguments $(__docker_arguments) \
+                $opts_help \
+                "($help -f --force)"{-f,--force}"[Force removal]" \
+                "($help)--no-prune[Do not delete untagged parents]" \
+                "($help -)*: :__docker_complete_images" && ret=0
             ;;
         (save)
             __docker_subcommand && ret=0
@@ -2222,11 +2225,8 @@ __docker_subcommand() {
             __docker_image_subcommand && ret=0
             ;;
         (rmi)
-            _arguments $(__docker_arguments) \
-                $opts_help \
-                "($help -f --force)"{-f,--force}"[Force removal]" \
-                "($help)--no-prune[Do not delete untagged parents]" \
-                "($help -)*: :__docker_complete_images" && ret=0
+            words[1]='rm'
+            __docker_image_subcommand && ret=0
             ;;
         (save)
             _arguments $(__docker_arguments) \