dev-install: drop unused make.conf.user

This file hasn't been used in a long time (probably not since CL:4060
where we switched to make.defaults).  Drop it to clean things up.

BUG=chromium:935142
TEST=precq passes

Change-Id: I3c56e9b875e7664d7e7e6fd39788d4ab796c985f
Reviewed-on: https://chromium-review.googlesource.com/1484469
Commit-Ready: Mike Frysinger <vapier@chromium.org>
Tested-by: Mike Frysinger <vapier@chromium.org>
Reviewed-by: Ned Nguyen <nednguyen@google.com>