Remove repos.conf

Portage will, by default, install repos.conf in /usr/share/portage/config. This
repos.conf defines a default repo (gentoo) in /usr/portage.
As this repo does not exist for Chromium OS, it throws a lot of warning on every
portage commands (emerge, equery, portageq, etc...).

BUG=None
TEST=./build_packages --board=amd64-generic does not show warnings.

Change-Id: I6d6118916441ed705328b097faa0dd59d344a878
Reviewed-on: https://chromium-review.googlesource.com/214827
Tested-by: Bertrand Simonnet <bsimonnet@chromium.org>
Reviewed-by: Mike Frysinger <vapier@chromium.org>
diff --git a/Makefile b/Makefile
index 92ea195..babbda2 100644
--- a/Makefile
+++ b/Makefile
@@ -27,7 +27,7 @@
 EXEMODE = 0755
 DIRMODE = 0755
 SYSCONFDIR_FILES = etc-update.conf dispatch-conf.conf
-PORTAGE_CONFDIR_FILES = make.conf.example make.globals repos.conf
+PORTAGE_CONFDIR_FILES = make.conf.example make.globals
 LOGROTATE_FILES = elog-save-summary
 BINDIR_FILES = ebuild egencache emerge emerge-webrsync \
 	emirrordist portageq quickpkg repoman