Remove enable/disable wheel gestures setting.

Since wheel gesture based scrolling has shipped and the code has been
removed from blink we can remove the runtime setting.

BUG=598798
CQ_INCLUDE_TRYBOTS=tryserver.blink:linux_blink_rel

Committed: https://crrev.com/3f5cea527647ebc8c505b1932d680854e162d82f
Review-Url: https://codereview.chromium.org/2047093002
Cr-Original-Commit-Position: refs/heads/master@{#398716}
Cr-Commit-Position: refs/heads/master@{#398862}
38 files changed