Remove res-560dpi symlink.

The change to platform/build in [1] added support for mapping numeric
DPIs to a proper density bucket. As a result, 560dpi gets mapped to
xxxhdpi automatically. This CL removes the previous workaround.

[1] https://android-review.googlesource.com/c/platform/build/+/741666

Bug: 111559919
Test: Build recovery image for marlin. Verify that it picks up xxxhdpi
      recovery resources.
Test: Boot into recovery. Run graphics test.
Change-Id: I65fefc7e2ba90b347e0ad283139429266d77c3b4