.circleci: don't use 'brew prune'

In the latest release of Homebrew (which Circle CI has upgraded to for
us) the command `brew prune` has been removed.

In order to stop failing our Circle CI builds, let's remove this
invocation.

The recommended alternative is to run 'brew cleanup', but this is
unnecessary: Homebrew 2.0 already does this for us, so repeating it here
is unnecessary.

.circleci/config.yml

@@ -8,7 +8,6 @@ jobs:
    steps:
      - checkout
      - run: brew update
      - run: brew prune
      - run: brew upgrade go || brew install go
      - run: brew upgrade git || brew install git
      - run: brew upgrade gettext || brew install gettext