Revert "add a dependabot.yml to checkout github actions versions (AMR…

…eX-Codes#3704)" (AMReX-Codes#3710)

This reverts commit 53a581c.

It creates too much noise for forks, and it cannot be disabled for
forks.

Another issue. It appears that it just called docs.yml and push a new
branch "main" to AMReX-Codes/amrex, not
AMReX-Codes/AMReX-Codes.github.io.