make_chroot: clear existing make.conf.user

We've been creating this as a self-referential symlink which means
we've been creating this file in the host distro.  Nuke the file
before we write to it.

BUG=b:191307774
TEST=`cros_sdk` still works

Change-Id: Ia07d1d4eeeb5bfabde07fb96e27d50fb5438e1b9
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/crosutils/+/2986861
Tested-by: Mike Frysinger <vapier@chromium.org>
Reviewed-by: Alex Klein <saklein@chromium.org>
Commit-Queue: Mike Frysinger <vapier@chromium.org>