maint/CICD ~ remove Dependabot configuration to avoid issue spam (in …

…main and forks)

# [why]

When using the Dependabot configuration (*.github/dependabot.yml*), forks are forced into
using it as well which causes lots of useless Dependabot spam. There is discussion but no
fix for this as of yet.

- ref: [Dependabot config and forks](dependabot/dependabot-core#2198)