CheckTrailingGarbage was called even when "-passfile" was
used, which is stupid, and causes false positives:
https://github.com/rfjakob/gocryptfs/issues/391
(false error "Received trailing garbage after the password"
when using -passfile in .bash_profile)
Instead of trying to improve the logic to handle that case
and make everything even more complicated, delete the function.
It is unclear if actually helps in some cases, and it definitely
harms as shown by the above bug report.
fe06e9f456