ci: build.sh: Drop the commentary about CI_BUILD_SCRIPT

ci: build.sh: Drop the commentary about CI_BUILD_SCRIPT

build.sh is not the place where this should be mentioned as the
official entrypoint for this script locally is ci/helper which can
download the right image from our upstream CI registry. Since the idea
is to ultimately drop the usage of a Makefile for the local executions,
this patch doesn't provide an alternative place for the comment in
question as the functionality is going to be altered substantially in
the future.

Signed-off-by: Erik Skultety <eskultet@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>