Remove Jitsu-related frontend code from Product Analytics features

What does this MR do and why?

Remove jitsu code and feature flag logic from the frontend:

• onboarding flow
• settings pages
• dashboards list

Note there is further Jitsu code to be removed in a subsequent MR, mostly from the dashboard page itself.

Product analytics is currently an experimental feature so these changes will not impact customers at this stage.

Screenshots or screen recordings

All before pics have product_analytics_snowplow_support === false so that jitsu is used. After, the flag is not checked either way.

before	after
project settings	project settings
	admin settings

Dashboards list (no visual difference)

How to set up and validate locally

1. Follow these instructions to setup Product Analytics in GDK.
2. Ensure that product_analytics_snowplow_support feature flag is disabled
3. Verify that Jitsu is not mentioned anywhere and all functionality works with Snowplow as expected:

• onboarding flow
• project settings
• admin settings
• dashboard list

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

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

Related to #406660 (closed)