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Removes these flags as they are enabled by default on stable already
and we won't disable them anymore:
 - ClickToCallContextMenuForSelectedText
 - SharingDeviceRegistration
 - ClickToCallOpenDialerDirectly
The ClickToCallReceiver feature is kept as a kill switch but the chrome
flags entry for it is removed.

Bug: None
Change-Id: I05a5f10325176c56ecaaf6419d3c876675eced3a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1926627
Commit-Queue: Richard Knoll <knollr@chromium.org>
Reviewed-by: Michael van Ouwerkerk <mvanouwerkerk@chromium.org>
Reviewed-by: Henrique Nakashima <hnakashima@chromium.org>
Cr-Commit-Position: refs/heads/master@{#717678}