Remove reference_static_offsets used for iterating through class static fields.

Since static fields are contiguous in class object and there is no need to
traverse super classes, it's not meaningful to use reference_static_offsets.
Also especially with embedded vtable/imt, static field offset can't be encoded
with an unsigned integer anyway.

A corresponding change is made to Class.java to remove the member field.

Bug: 16236588
Change-Id: I1fde3cd9efce884945876f0658c63d992164fd94