dhall freeze: Remove cache import expression before a secure freeze

[mmhat]

Doing so allows us to protected imports securely that were previously only protected for caching purposes. For example, given the following Dhall file foo.dhall:

  missing
    sha256:9bb9dcb5bf6f795291686f59383bcd01c8e79b87fc3fb63351d46dea100ac51b
? ./bar.dhall

Previously dhall freeze --all foo.dhall threw an "No valid imports" error as it was failing to add a hash to the missing import. With the changes in this PR freezing foo.dhall results in the following expected content of that file:

./bar.dhall
  sha256:9bb9dcb5bf6f795291686f59383bcd01c8e79b87fc3fb63351d46dea100ac51b