Remove synchronized & rest of accessible flag during injection

To improve multithreaded performance of Sling Models, the synchronized blocks, along with the "reset" of the setAccessible using its original value should be removed.

Context:
The synchronized blocks were added to resolve a race condition [1]. After looking into another large project that uses reflection to access members of classes (Felix [2]), nowhere in that project is something similar being done. Every reflective access is just doing setAccessible(true).

Results from a JMH test allude to a significant performance improvement during multithreaded threaded access by removing the synchronized (JMH results [3]).

[1]: SLING-6584
[2]: https://github.com/apache/felix-dev/search?q=setAccessible
[3]: https://gist.github.com/paul-bjorkstrand/f3bb154665e7d2168b4656eb7b794496
[4]: https://www.mail-archive.com/dev@sling.apache.org/msg113123.html
[5]: https://github.com/apache/sling-org-apache-sling-models-impl/pull/11