Revert "Add test case to detect dodgy b64 encoding"

This reverts commit 5fe76b9434.

I think I had this accidentally commited on my local develop branch, and
so it accidentally got merged into upstream develop.

This should re-land with corrections in #16504.

tests/http/test_proxyagent.py

@@ -217,20 +217,6 @@ class ProxyParserTests(TestCase):
         )
 
 
-class TestBasicProxyCredentials(TestCase):
-    def test_long_user_pass_string_encoded_without_newlines(self) -> None:
-        """Reproduces https://github.com/matrix-org/synapse/pull/16504."""
-        creds = BasicProxyCredentials(
-            b"looooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooonguser:pass@proxy.local:9988"
-        )
-        auth_value = creds.as_proxy_authorization_value()
-        self.assertNotIn(b"\n", auth_value)
-        self.assertEqual(
-            creds.as_proxy_authorization_value(),
-            b"Basic: bG9vb29vb29vb29vb29vb29vb29vb29vb29vb29vb29vb29vb29vb29vb29vb29vb29vb29vb29vb29vb29vb29vb29vb29vbmd1c2VyOnBhc3M=",
-        )
-
-
 class MatrixFederationAgentTests(TestCase):
     def setUp(self) -> None:
         self.reactor = ThreadedMemoryReactorClock()