docs: remove nonexistent CAMOFOX_PROFILE_DIR env var references (#10976)

Camofox automatically maps each userId to a persistent Firefox profile
on the server side — no CAMOFOX_PROFILE_DIR env var exists. Our docs
incorrectly told users to configure this on the server.

Removed the fabricated env var from:
- browser docs (:::note block)
- config.py DEFAULT_CONFIG comment
- test docstring

tests/tools/test_browser_camofox_persistence.py

@@ -1,8 +1,8 @@
 """Persistence tests for the Camofox browser backend.
 
 Tests that managed persistence uses stable identity while default mode
-uses random identity. The actual browser profile persistence is handled
-by the Camofox server (when CAMOFOX_PROFILE_DIR is set).
+uses random identity. Camofox automatically maps each userId to a
+dedicated persistent Firefox profile on the server side.
 """
 
 import json