Remove appengine/swarming from .gitignore.
Remove appengine/swarming from .gitignore.

It was forgotten from 4392884e2.

R=vadimsh@chromium.org
BUG=

Review URL: https://codereview.chromium.org/1087313003

nicko96/Chrome-Infra,nicko96/Chrome-Infra,nicko96/Chrome-Infra,nicko96/Chrome-Infra,nicko96/Chrome-Infra