Fix Dependabot config (#1911)

It was previously complaining:

> Your .github/dependabot.yml contained invalid details
>
> Dependabot encountered the following error when parsing your
> .github/dependabot.yml:
>
> ```
> Update configs must have a unique combination of 'package-ecosystem',
> 'directory', and 'target-branch'. Ecosystem 'cargo' has overlapping
> directories.
> ```
>
> Please update the config file to conform with Dependabot's
> specification.

Author: tarcieri

.github/dependabot.yml

@@ -1,11 +1,6 @@
 version: 2
 updates:
-- package-ecosystem: cargo
-  directory: "/"
-  schedule:
-    interval: weekly
-  open-pull-requests-limit: 10
-- package-ecosystem: cargo
+ package-ecosystem: cargo
   versioning-strategy: lockfile-only
   directory: "/"
   allow: