Remove support for missing <scm> section

[basil]

It turned out that there were a handful (less than 5) of plugins that were missing an SCM section, resulting in #451. It would be ideal to fix those plugins and then to rip out support for plugins without an SCM section along with the second half of this if statement:

    public static String getScmTag(PomData pomData, String name, String version) {
        String scmTag;
        if (pomData.getScmTag() != null) {
            scmTag = pomData.getScmTag();
            LOGGER.log(Level.INFO, "Using SCM tag {0} from POM", scmTag);
        } else {
            scmTag = name + "-" + version;
            LOGGER.log(Level.INFO, "POM did not provide an SCM tag; inferring tag {0}", scmTag);
        }
        return scmTag;
    }

This cleanup would then cascade into the inlining of this function and the removal of the getParentProjectName() method, of which this is the only consumer.

The reason it would be ideal is that it is not very maintainable to have to maintain two code paths when the second is so rarely used.