__groovy function seems to have useless synchronized in execute

@pmouawad ([Bug 62082](https://bz.apache.org/bugzilla//show_bug.cgi?id=62082&redirect=false)):
Following same analysis of https://github.com/apache/jmeter/issues/3460 and as per understanding of this:

- https://stackoverflow.com/questions/48644849/is-groovy-scriptingengine-thread-safe

Groovy ScriptEngine "THREADING" is equal to MULTITHREADED:
- https://docs.oracle.com/javase/8/docs/api/javax/script/ScriptEngineFactory.html#getParameter-java.lang.String-

It seems we could avoid the synchronized block.

Thoughts ?
Thanks

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