LRDOCS-1410 For simplicity's sake, removed support for multiple types of sidebars

https://issues.liferay.com/browse/LRDOCS-1410

For simplicity's sake, removed support for multiple types of sidebars.

Note: I haven't tested this commit since ant jar no longer generates the markdown-converter.jar file. Apparently, the liferay-portal/tools/sdk folder is gone?

The liferay-portal/modules/util/markdown-converter/build.xml references liferay-portal/tools/sdk/build-common-osgi-plugin.xml, which no longer exists.