Remove unused premature_close server endpoint

Last usage was removed in encode#804

tests/conftest.py

@@ -76,8 +76,6 @@ async def app(scope, receive, send):
     assert scope["type"] == "http"
     if scope["path"].startswith("/slow_response"):
         await slow_response(scope, receive, send)
-    elif scope["path"].startswith("/premature_close"):
-        await premature_close(scope, receive, send)
     elif scope["path"].startswith("/status"):
         await status_code(scope, receive, send)
     elif scope["path"].startswith("/echo_body"):
@@ -113,16 +111,6 @@ async def slow_response(scope, receive, send):
     await send({"type": "http.response.body", "body": b"Hello, world!"})
 
 
-async def premature_close(scope, receive, send):
-    await send(
-        {
-            "type": "http.response.start",
-            "status": 200,
-            "headers": [[b"content-type", b"text/plain"]],
-        }
-    )
-
-
 async def status_code(scope, receive, send):
     status_code = int(scope["path"].replace("/status/", ""))
     await send(