ci: remove dependabot config

Bot was generating mostly-wrong upgrade PRs that don't understand the
crypto stack's transitive pinning constraints. We'll bump deps by hand
when there's reason to.

.github/dependabot.yml

@@ -1,21 +0,0 @@
-version: 2
-updates:
-  - package-ecosystem: cargo
-    directory: /
-    schedule:
-      interval: weekly
-    open-pull-requests-limit: 5
-    groups:
-      patch-updates:
-        update-types: [patch]
-
-  - package-ecosystem: cargo
-    directory: /fuzz
-    schedule:
-      interval: monthly
-    open-pull-requests-limit: 2
-
-  - package-ecosystem: github-actions
-    directory: /
-    schedule:
-      interval: weekly