Revert "Wrap 'u_rtFlip.y * dfdy()' in parentheses."

This reverts commit 9fc189f1cbdfa0b8a9255e18a79772ced60dbfde.

Reason for revert: shader compile failure on AndroidOne-GPU-Mali400MP2 devices

Original change's description:
> Wrap 'u_rtFlip.y * dfdy()' in parentheses.
>
> This guards against unexpected results when dfdy is used in complex
> expressions. In practice, I'm not aware of this causing any trouble.
>
> Change-Id: I58d4762871481fdb4c173b570e4d5d6edf657af7
> Reviewed-on: https://skia-review.googlesource.com/c/skia/+/465077
> Reviewed-by: Ethan Nicholas <ethannicholas@google.com>
> Commit-Queue: Ethan Nicholas <ethannicholas@google.com>
> Auto-Submit: John Stiles <johnstiles@google.com>

Change-Id: Idfaa9316d657717d5ee7117837c9cc9c3d4ee189
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/465377
Auto-Submit: Greg Daniel <egdaniel@google.com>
Commit-Queue: Rubber Stamper <rubber-stamper@appspot.gserviceaccount.com>
Bot-Commit: Rubber Stamper <rubber-stamper@appspot.gserviceaccount.com>
4 files changed