mount-chroot: Remove unecessary ecryptfs options

- `no_sig_cache` is normally interpreted by the mount helper
  (`mount.ecryptfs`), but we use `-i` in the `mount` command, so
  the helper never gets called. (the man page is incorrect, it's
  not a kernel option) So we can just remove it.
- `ecryptfs_passthrough` does not take a `=y/n` parameter. Either
  you set it (`ecryptfs_passthrough`), or you don't (in that case
  the default behaviour is `=n` as intended). I checked Chromium
  kernel sources 3.4 to 3.14, same behaviour on all of them.
  Tested on a 3.8 machine: I removed the option, dropped a file
  in, it appears, but cat-ing the file results in input-output
  error.

Removes spurious dmesg errors, fixes #1122.