Revert "Fix String.equals() for moveable String.class."

This reverts commit da283050.

Reason for revert: This CL is no longer needed with

  https://android-review.googlesource.com/737437

which makes the String class non-movable in all cases.

Test: art/test.py
Bug: 68181300
Bug: 67628039
Change-Id: I414abd79de1e1b0fc43dc3be412fc70598ef3044