scripts: dbgmtree.py: Dropped Mtree.lookupnext

I was toying with making this look more like the mtree API in lfs.c (so
no lookupleaf/namelookupleaf, only lookup/namelookup), but dropped the
idea:

- It would be tedious

- The Mtree class's lookupleaf/namelookupleaf are also helpful for
  returning inner btree nodes when printing debug info

- Not embedding mids in the Mdir class would complicate things

It's ok for these classes to not match littlefs's internal API
_exactly_. The goal is easy access for debug info, not to port the
filesystem to Python.

At least dropped Mtree.lookupnext, because that function really makes no
sense.

scripts/dbgbmap.py

@@ -1883,40 +1883,6 @@ class Mtree:
         else:
             return mdir
 
-    def lookupnext(self, mid, *,
-            path=False,
-            depth=None):
-        if not isinstance(mid, Mid):
-            mid = self.mid(mid)
-
-        # lookup the relevant mdir
-        r = self.lookupleaf(mid,
-                path=path,
-                depth=depth)
-        if path:
-            mdir, path_ = r
-        else:
-            mdir = r
-        if mdir is None:
-            if path:
-                return None, None, path_
-            else:
-                return None, None
-
-        # not in mdir?
-        if mid.mrid >= mdir.weight:
-            if path:
-                return None, None, path_
-            else:
-                return None, None
-
-        # lookup mid in mdir
-        rattr = mdir.lookupnext(mid)
-        if path:
-            return mdir, rattr, path_+[(mid, mdir, rattr)]
-        else:
-            return mdir, rattr
-
     def lookup(self, mid, *,
             path=False,
             depth=None):