Stale nitpick_ignore entry for deleted controller_pv_prefix in docs/conf.py

[gilesknap]

Found while reviewing PR #360 commit c8adb33b (EPICS CA multi-root softioc with id-based PV prefix).

docs/conf.py:101 still carries:

("py:obj", "fastcs.transports.epics.util.controller_pv_prefix"),

in nitpick_ignore, but controller_pv_prefix was removed from src/fastcs/transports/epics/util.py in the same commit. The ignore entry now suppresses warnings for a symbol that no longer exists — dead config.

Trivial cleanup: drop that line.

Filed by: PR #360 commit-by-commit review.