Add "ostree remote delete" and corresponding API

For Anaconda, we have an ugly bootstrapping problem where we need to
add the remote to the repository's config, then do a pull+deploy, then
remove and re-add the config, because /etc/ostree/remotes.d doesn't
exist yet in the target system.

https://bugzilla.gnome.org/show_bug.cgi?id=738698

tests/test-remote-add.sh

@@ -32,3 +32,13 @@ echo "ok config"
 $OSTREE remote add --no-gpg-verify another http://another.com/repo
 assert_file_has_content $test_tmpdir/repo/config "gpg-verify=false"
 echo "ok remote no gpg-verify"
+
+$OSTREE remote delete another
+echo "ok remote delete"
+
+if $OSTREE remote delete nosuchremote 2>err.txt; then
+    assert_not_reached "Deleting remote unexpectedly succeeded"
+fi
+assert_file_has_content err.txt "error: "
+
+