interfaces: drop `_ignorefiles()` from `dirstate` Protocol

I didn't look back at the state of things when this was introduced as a zope
interface, but the method is only called inside of `mercurial.dirstate.dirstate`
now.

In general, private methods shouldn't be in the Protocol class, but some of
these are called from outside `dirstate` by debug commands.

mercurial/interfaces/dirstate.py

@@ -110,9 +110,6 class idirstate(Protocol):
     def is_changing_files(self) -> bool:
         """True if file tracking changes in progress."""
 
-    def _ignorefiles(self) -> List[bytes]:
-        """Return a list of files containing patterns to ignore."""
-
     def _ignorefileandline(self, f: bytes) -> IgnoreFileAndLineT:
         """Given a file `f`, return the ignore file and line that ignores it."""