dev-install: drop custom make.conf file

Now that portage is able to load the settings it needs without this
config file (since /usr/share/portage is in the base image), drop it
from the install.

BUG=chromium:935142
TEST=dev_install still works
CQ-DEPEND=CL:1495194, CL:1497670

Change-Id: I801bae85d9363404020fa8449a90c60732d4d1dd
Reviewed-on: https://chromium-review.googlesource.com/1497667
Commit-Ready: Mike Frysinger <vapier@chromium.org>
Tested-by: Mike Frysinger <vapier@chromium.org>
Reviewed-by: Amin Hassani <ahassani@chromium.org>
2 files changed