Remove obsolete cross-port script

This patch removes the obsolete cross-port script in the tree. The
script was based on the legacy Make build system which has been pruned
from the tree long ago. For hadrian, proper support for two-stage
bootstrapping onto a new unsupported platform is a work in progress in
!11444.

(cherry picked from commit 00d551bf)
(cherry picked from commit 329a26da)