Enable Reset password by phone only if `RealNameSystem` is enabled

What does this MR do and why?

Resolve potentially HK SaaS problem about reset password phone from !1320 (merged).

It has known that HK SaaS won't force user have verified-phone in their account. So it will be better to disable reset password by phone feature if thus happen.

also related to #2049 (closed) and comment from PM here

Screenshots or screen recordings

Screenshots are required for UI changes, and strongly recommended for all other merge requests.

How to set up and validate locally

Enable RealNameSystem:

# switch into rail console: rails console
setting = ::Gitlab::CurrentSettings.current_application_settings
setting.phone_verification_code_enabled = true
setting.save!

Numbered steps to set up and validate the change are strongly suggested.

MR acceptance checklist

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• I have evaluated the MR acceptance checklist for this MR.