Simplify check for changing UserId

[sgiehl]

Description:

Technically this change should not have any effect.
If enable_userid_overwrites_visitorid is enabled, a new visitorid will be set anyway when the userid changes. This also causes a new visit, as the visitorid then always differs from the previous one.

But as we always want to force a new visit when the user id changes, the config check shouldn't be done here at all.

refs #19927

Review

• Functional review done
• Potential edge cases thought about (behavior of the code with strange input, with strange internal state or possible interactions with other Matomo subsystems)
• Usability review done (is anything maybe unclear or think about anything that would cause people to reach out to support)
• Security review done
• Wording review done
• Code review done
• Tests were added if useful/possible
• Reviewed for breaking changes
• Developer changelog updated if needed
• Documentation added if needed
• Existing documentation updated if needed