vm: Remove unneeded dependency on Guile-JSON.

Fixes <https://bugs.gnu.org/31581>.
Reported by Leo Famulari <leo@famulari.name>.

* gnu/system/vm.scm: Remove unneeded (guix docker) import.

gnu/system/vm.scm

@@ -23,7 +23,6 @@
 
 (define-module (gnu system vm)
   #:use-module (guix config)
-  #:use-module (guix docker)
   #:use-module (guix store)
   #:use-module (guix gexp)
   #:use-module (guix derivations)