[CI] Remove deprecated aarch64 GitHub Actions workflow

* The aarch64 GitHub Actions workflow file (.github/workflows/aarch64.yml) was deleted.

* This action is no more needed as the ci.yml works fine with Nix-Magic-Cache

* Rather than building 2 derivation i.e. otb-dev (all modules) and otb (minimal modules) which takes a lot of time for ARM64 build and gets timed out so we build only one derivation i.e otb-dev which contains all the modules.