makefiles/boot.zuo: remove broken main submodule

The main submodule hasn't worked since at least commit 10d59cc, and
documented build modes have been added in the mean time that support
its former uses.

Closes #810

makefiles/boot.zuo

@@ -6,19 +6,6 @@
          cross-build-boot
          clean-links)
 
-(module+ main
-  (command-line
-   :args (scheme machine)
-   (lambda (accum)
-     (define vars (hash))
-     (cross-build-boot #f (hash) '("all")
-                       machine
-                       (if (car (split-path scheme))
-                           scheme
-                           (find-executable-path scheme))
-                       #f
-                       (make-at-dir "xc") vars vars #f))))
-
 (define boot-file-names
   '(;; the boot files proper
     "petite.boot" "scheme.boot"