remove --enable-preview from microbenchmarks

See comment added to PR [1]: by Peter Levart:

Hi @vicente-romero-oracle , note that besides tests, there is also a JMH benchmark that measures the performance of records deserialization (org.openjdk.bench.java.io.RecordDeserialization) which forced us to modify the build procedure for all benchmarks to include --enable-preview option in JDK 15 and backports (see https://bugs.openjdk.java.net/browse/JDK-8248135). If you undo this change in JDK 16 then also the problem described in https://bugs.openjdk.java.net/browse/JDK-8250669 and https://bugs.openjdk.java.net/browse/JDK-8248429 will disapear. After that, perhaps undoing the same for JDK 15 and backports together with removing the benchmark is also possible to resolve the issues in older releases as most developement will probably happen in JDK 16 then and so the need for performance testing will mostly be needed in there.
We still have to figure out how to enable having benchmarks for preview features as in the future there most probably will be a need for that.

[1] https://github.com/openjdk/jdk/pull/290