java: remove forced synchronization

Removes the synchronized keyword from the JNI entry points. Libcephfs
should already be thread-safe.

Signed-off-by: Noah Watkins <noahwatkins@gmail.com>
java: remove mount() synchronization

Not protecting anything. Synchronization is applied at the native
interface level.

Signed-off-by: Noah Watkins <noahwatkins@gmail.com>