settings: Remove 'useful for testing' from label.
settings: Remove 'useful for testing' from label.

This label is unnecessary and creates the potential for an incorrect
assumption that it's only useful for testing, when in fact it's a
reasonable configuration overall.

zulip/zulip,zulip/zulip,zulip/zulip,zulip/zulip,zulip/zulip,zulip/zulip,zulip/zulip