maint(resources): remove Keyman 18 steps of TC build configs

With #13399 we added new build steps to the TC build configurations that make use of TC scripts in `resources/teamcity`. We kept the old steps so that we're able to continue to build Keyman 18.

After the release of Keyman 19 we should remove those build steps, and also remove the first lines of the new build steps that will skip this step if building Keyman 18.