[Lilliput/JDK17] Remove wrong assert in klass.hpp

In klass.hpp, Klass::prototype_header() we have an assert(UseCompactObjectHeaders) which came from the similar upstream jdk21 change, but is wrong in jdk17, because the prototype_header() will also be used when biased locking is used. Biased locking is disabled by default, and the assert is harmless in release builds, but it makes a bunch of tests fail. Let's remove the assert and reduce upstream-jdk17u diff a little bit.