core#2063: Don't throw a warning when public ext directory isn't found

[MegaphoneJon]

https://lab.civicrm.org/dev/core/-/issues/2063

Overview

When the extension directory feed is offline, a warning is thrown in CiviCRM. However, this warning isn't actionable by the Civi administrator, and in 2020 this will never happen unless the directory is temporarily offline.

Before

There are currently no extensions on the CiviCRM public extension directory which are compatible with version 5.29.1. If you want to install an extension which is not marked as compatible, you may be able to download and install extensions manually (depending on access to your web server).

After

Silence

[civibot]

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[eileenmcnaughton]

Looking at gitlab it seems @colemanw previously approved this & seems straight forward. Since I'm doing the dev-digest I'll include this but if no negative reaction can merge in a couple of days

[eileenmcnaughton]

No reaction from the dev-digest -merging