Revert JDK GCM direct buffer crash workaround

Motivation:
Commit 108dc23c introduced a workaround due to a JDK crash when GCM cipher was used during an unwrap operation. Attempting to reproduce this issue with the latest JDK (1.8.0_72-b15) demonstrate that this issue no longer exists while it can be reliably reproduced on earlier JDKs (1.8.0_25-b17 and earlier)

Modifications:
- Remove the copy-to-heap-buffer workaround for JDK engine

Result:
Fixes https://github.com/netty/netty/issues/3256