Intent to remove pcast and feed protocols

[Jonathan Kingston]

We have a two feed handling protocols that were never standardised and
aren't implemented in other browsers. These protocols have been subject to
various security bugs and also contribute to some technical debt.

The protocols are used to route URLs in Firefox to the feed reader. The
feed reader won't be impacted by this change or other registered feed
handlers.

The work was implemented here:
https://bugzilla.mozilla.org/show_bug.cgi?id=1420622

I intend to fully remove this code and let it ride the trains.

As the telemetry suggests the pcast protocol is almost never used so is
safe to be removed for all users. The feed protocol however is used by the
"Subscribe to this page" functionality and was changed as part of this
patch to work without the protocol.

https://telemetry.mozilla.org/new-pipeline/dist.html#!cumulative=0&end_date=2017-11-28&keys=__none__!__none__!__none__&max_channel_version=release%252F57&measure=FEED_PROTOCOL_USAGE&min_channel_version=null&processType=*&product=Firefox&sanitize=1&sort_keys=submissions&start_date=2017-11-12&table=0&trim=1&use_submission_date=0

<https://telemetry.mozilla.org/new-pipeline/dist.html#!cumulative=0&end_date=2017-11-28&keys=__none__!__none__!__none__&max_channel_version=release%252F57&measure=FEED_PROTOCOL_USAGE&min_channel_version=null&processType=*&product=Firefox&sanitize=1&sort_keys=submissions&start_date=2017-11-12&table=0&trim=1&use_submission_date=0>
Shout out to :bz who fixed a test that was depending on the feed protocol.

Thanks
Jonathan