Revert "Make scrolling in PanelLayout smoother on iOS"

This reverts commit 5708bdf45cf3f7ade3b93d0b2feae2f23b328d16.

I was too hasty when introducing this change. Apparently iOS Safari
freaks out when there is another element with -webkit-overflow-scrolling: touch.
We use it for scrolling the editor surface already and this causes the
editor to become totally unscrollable and unfocusable after closing a
dialog.

Bug: 68878
Change-Id: If51946906e3a91e30a718b81c83bb94830bc921b
Merge "Revert "Make scrolling in PanelLayout smoother on iOS""