[Github] Delete dependabot config (#158337)

Dependabot cannot configure the branch prefix, which means it fails
everytime it tries to run because we only allow user/ branches.

This is in preparation for using Renovate which supports custom branch
prefixes and has other advantages, like the ability to run/get setup
without any assisstance from a repository admin unlike dependabot. This
makes it significantly more hackable for the rest of the community.

.github/dependabot.yml

@@ -1,18 +0,0 @@
-version: 2
-updates:
-  - package-ecosystem: "github-actions"
-    directory: "/"
-    schedule:
-      interval: "monthly"
-    groups:
-      github-actions:
-        patterns:
-          - "*"
-  - package-ecosystem: "pip"
-    directory: "/llvm/docs"
-    schedule:
-      interval: "monthly"
-    groups:
-      llvm-docs-requirements:
-        patterns:
-          - "*"