Linkage Monitor fails to detect linkage errors in gax-java.

Linkage Monitor has capability to replace a BOM in the libraries BOM. However, we no longer specify gax-bom in it. Therefore current it does not detect anything in gax-java's repository.

This bug is only applicable for gax-java repository, which relies on gax-java BOM to tell locally installed artifacts. Other Maven projects in Yoshi repositories, which produces pom.xml files in project directories, are not affected.