travis/gitlab/azure: Drop BUILDMAN variable with test.py

This is not needed in the test.py part of the config, now since we use the
same name as the pytests.

Drop BUILDMAN, retaining it only for the 'build' parts of the config, i.e.
where we build multiple boards and don't run any tests.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Tom Rini <trini@konsulko.com>