gles2n64: Fixed frameskip-config regression.

This commit fixes Issue #272.

Auto-frameskip regressed in commit
ae8db02bbe96d842809f8e007557da3654a08496

Basically, that commit made gles2n64 use an obsolete code path whenever
auto-frameskip was enabled.  In that commit I changed how the frameskip
setting was read, using the config file rather than JNI. In that commit
I created a new config file field (maxFrameskip) but reused an existing
one (autoFrameskip). But that was a mistake because the autoFrameskip
field was already being used, in a way that had an entirely different
meaning.  Prior to that commit, autoFrameskip always had a value of 0
(since we didn't explicitly set it) which made gles2n64 use Yongzh's
FrameSkipper class (which encapsulates both auto and manual skipping).
When autoFrameskip was set to 1, gles2n64 used some legacy frameskip
algorithm that Yongzh's FrameSkipper class superseded.  So basically I
hijacked an existing flag and tried to give it a new meaning without
understanding its original meaning.

This commit fixes the regression.  To correct the problem and avoid any
further confusion, I just removed the legacy autoframeskip algorithm
altogether, always using Yongzh's much better algorithm, which is
encapsulated in the FrameSkipper class.