chore(dashboards): remove text widget flag references frontend

[nikkikapadia]

more text widgets for everyone YAY

should be merged after #116207

[github-actions]

📊 Type Coverage Diff

✅ No new type safety issues introduced. Coverage: 93.56%

[sentry]

Bug: The frontend allows creating text widgets unconditionally, but the backend still rejects them for users without the dashboards-text-widgets feature flag, causing an error on save.
_{Severity: MEDIUM}

Suggested Fix

To resolve the mismatch, either the backend validation for the dashboards-text-widgets feature flag should be removed to align with the frontend, or the frontend should be updated to conditionally render the text widget option based on the presence of the feature flag.

Prompt for AI Agent

Review the code at the location below. A potential bug has been identified by an AI
agent. Verify if this is a real issue. If it is, propose a fix; if not, explain why it's
not valid.

Location: static/app/views/dashboards/widgetBuilder/components/typeSelector.tsx#L86-L90

Potential issue: The frontend code in `typeSelector.tsx` was changed to unconditionally
show the "Text (Markdown)" widget option to all users. However, the backend still
enforces the `dashboards-text-widgets` feature flag. This mismatch means that users who
do not have the feature flag enabled will see the option to create a text widget,
configure it, but will receive a 400 validation error when they try to save it. The
backend rejects the request with the error "Text widgets are not enabled", leading to a
confusing and broken user experience.

Also affects:

• src/sentry/api/serializers/rest_framework/dashboard.py:466-473
• chart.tsx:494-498

_{Did we get this right? 👍 / 👎 to inform future reviews.}