chore(flags): Remove `is_simple_flag` from everything

[haacked]

The is_simple_flag property is no longer returned by any of the flags related API. However, there's still a lot of references to it in our code and lots of client SDK code that references it.

We should remove all the code that references it and then eventually even remove the column. This is a tracking issue for the necessary changes. Newer clients never even referenced this property, so I just need to confirm that here:

• posthog-js
• posthog-js-lite - chore: remove deprecated is_simple_flag from PostHogFeatureFlag posthog-js-lite#516
• posthog-python
• posthog-go chore: Remove isSimpleFlag related code posthog-go#88
• posthog-ruby
• posthog-java
• posthog-dotnet
• posthog-elixir
• posthog

[ted-mundy]

@haacked I can't seem to see any references to is_simple_flag or isSimpleFlag in posthog-js. Seems like it's already been taken out?

[haacked]

@ted-mundy thanks for checking!

[KreativeThinker]

Completed for posthog-python as per changelog

## 3.12.1 - 2025-02-11

1. Fix mypy support for 3.12.0
2. Deprecate `is_simple_flag`

Declaration remaining in tests posthog-python/posthog/test/test_feature_flags.py

                "is_simple_flag": True,

[jerrylin3321]

Don't see isSimpleFlag in the Java client: https://github.com/search?q=repo%3APostHog%2Fposthog-java+isSimpleFlag&type=code

[luke-belton]

related #36753 - seeing is_simple_flag still set after changing a 'simple' flag to be multivariate