github: remove dependabot configuration

This does not seem to have worked at all. For example, there were
Actions being used that were clearly deprecated/archived[1]. But
Dependabot didn't make a peep. So just get rid of it to avoid the false
sense that someone is checking our dependencies for us.

[1]: https://github.com/BurntSushi/ripgrep/pull/2360

.github/dependabot.yml

@@ -1,6 +0,0 @@
-version: 2
-updates:
-  - package-ecosystem: "github-actions"
-    directory: "/"
-    schedule:
-      interval: "weekly"