[dagit] Remove clear-site-data header (#8134)

### Summary & Motivation

We don't really need this, there's useful stuff in our site data.

### How I Tested These Changes

Run dagit, verify that the header is not included in the response.

python_modules/dagit/dagit/webserver.py

@@ -77,7 +77,6 @@ def build_middleware(self) -> List[Middleware]:
     def make_security_headers(self) -> dict:
         return {
             "Cache-Control": "no-store",
-            "Clear-Site-Data": "*",
             "Feature-Policy": "microphone 'none'; camera 'none'",
             "Referrer-Policy": "strict-origin-when-cross-origin",
             "X-Content-Type-Options": "nosniff",