Bug 41259: Skip versions which don't set incremental_from when generating incrementals

Also don't set `incremental_from` for the legacy version in
update_responses config.

projects/release/update_responses_config.yml

@@ -77,7 +77,6 @@ versions:
         mar_channel_id: [% c('var/mar_channel_id') %]
         platformVersion: [% c('var/torbrowser_legacy_platform_version') %]
         detailsURL: https://blog.torproject.org/new[% IF c("var/alpha") %]-alpha[% END %]-release-tor-browser-[% c("var/torbrowser_legacy_version") FILTER remove('\.') %]
-        incremental_from: []
         # minSupportedOsVersion on macOS corresponds to the Darwin version ( https://en.wikipedia.org/wiki/Darwin_(operating_system) )
         macos:
             # macOS v10.12.0

tools/update-responses/update_responses

@@ -702,6 +702,7 @@ my %actions = (
         exit_error "Unknown channel" unless $config->{channels}{$channel};
         my $versions = as_array($config->{channels}{$channel});
         foreach my $version (@$versions) {
+            next unless $config->{versions}{$version}{incremental_from};
             extract_martools($config, $version);
             get_version_files($config, $version);
             create_incremental_mars_for_version($config, $version, $channel);