remove debs/tars extension check in _build_layer
remove debs/tars extension check in _build_layer

* extension check may silently filter things, which is dangerous

* extension was already enforced in the rule, so it is unnecessary to
check again:
```
    "tars": attr.label_list(allow_files = tar_filetype),
    "debs": attr.label_list(allow_files = deb_filetype),
```

bazelbuild/rules_docker,bazelbuild/rules_docker,bazelbuild/rules_docker,bazelbuild/rules_docker