Use NativeString from Gerrit core

In 2.11.2 NativeString in Gerrit core was fixed by [1]. Now we can
just use this class and get rid of the MyNativeString class.

[1] https://gerrit-review.googlesource.com/69180

Change-Id: I6dcd1b6fd49104c7ff19da862f05642be64299ea
Signed-off-by: Edwin Kempin <edwin.kempin@sap.com>