Oracle controls the "Java" trademark. They can prevent OpenJDK (and any other implementations) from being able to call future major versions Java (or indeed JVM/JRE/JDK), simply by not publishing or not licensing future versions of the TCK. (Licensing terms for the Java trademark require an implementation to pass the TCK to be able to call itself Java.)

Just removing the word Java from the OpenJDK sources and documentation would be a huge endeavor. All the standard tools ("java", "javac" etc) would have to be renamed. It might also require completely breaking backward compatibility, because APIs are now copyrighted and the Java standard library packages are named java.* (and probably there are class and method names mentioning "java" too).

ETA: even ignoring copyrights on API, the trademark itself might stop you from publishing the not-Java standard library under the package java.*.

ETA #2: other commenters say it's allowed to use a trademarked word as long as it's necessary for compatibility. But it's not allowed to actually say you're compatible. Apparently. I don't claim to understand this, but law is not require to make sense, so this could be true.