walker: Remove ignoreTrees functionality

It was only used in two places:
- stats: apparently as a minor performance optimization, which is
  unlikely to be important
- find: filtered directories would be ignored. However, this
  optimization missed that it is possible that two directories have the
  exact same content. Such directories would be incorrectly ignored too.
  Example:
```
mkdir test test/a test/b
restic backup test
restic find latest test/b
-> incorrectly does not return anything
```

Thus, remove the functionality as it's apparently too complex to use
correctly.

changelog/unreleased/pull-4615

@@ -0,0 +1,6 @@
+Bugfix: `find` ignored directories in some cases
+
+In some cases, the `find` command ignored empty or moved directories. This has
+been fixed.
+
+https://github.com/restic/restic/pull/4615