[Github] Delete dependabot config (llvm#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.

Author: boomanaiden154