Disable dependabot

I don't think this has ever actually sent a meaningful version bump PR
we weren't already aware of, so unless someone else wants to be in
charge of these I'm going to go ahead and disable dependabot.

.github/dependabot.yml

@@ -1,15 +0,0 @@
-version: 2
-updates:
-- package-ecosystem: cargo
-  directory: "/"
-  schedule:
-    interval: daily
-  open-pull-requests-limit: 10
-  reviewers:
-  - alexcrichton
-  assignees:
-  - alexcrichton
-  ignore:
-  - dependency-name: proptest
-    versions:
-    - ">= 0.9.a, < 0.10"