RFR: 8267578: Remove unnecessary preview checks

[Jonathan Gibbons]

Guoxiong,

To be clear, it is an overall requirement that in order to bootstrap-compile the JDK, we must be able to compile javac with the previously available release. Given the short interval between releases, this sometimes evaluates to a numerically 2-earlier release. For example, after we open up the repos for JDK 18 next month, JDK 17 will not be available until September, so we still need to compile javac with JDK 16 until then.

-- Jon