ci: fix Dependabot config (#620)

Unfortunately, we can not have two separate "jobs" for bumping direct
dependencies and for updating Cargo.lock. The old config results in the
following error:
>Update configs must have a unique combination of 'package-ecosystem',
'directory', and 'target-branch'. Ecosystem 'cargo' has overlapping
directories.

Author: newpavlov

.github/dependabot.yml

@@ -1,11 +1,5 @@
 version: 2
 updates:
-- package-ecosystem: cargo
-  directory: "/"
-  versioning-strategy: increase-if-necessary
-  schedule:
-    interval: daily
-  open-pull-requests-limit: 10
 - package-ecosystem: cargo
   directory: "/"
   versioning-strategy: lockfile-only