Remove legacy check for reversed ZWJ emoji sequences.

According to Behdad this issue was fixed in HarfBuzz 2.0.0, October
2018.

Change-Id: I6e23ae7d637f79cb55b99a73ac8d16a0b0395610

Bug: 158944323
Test: drew a ZWJ in RTL and it worked fine
(https://screenshot.googleplex.com/AHrRaKNR6S4Nwgb.png)

Change-Id: I29e64f4577c819dfec0f3cbd878f0241bf706de0