apache-httpd: Don't set default content encodings

In general, you don't want a .tar.gz file to be served with
"Content-Encoding: x-gzip", because this causes browsers (like Chrome
or "curl --compressed") to decompress the file on the fly. So you end
up with a .tar rather than .tar.gz file, which is unexpected.

If people want such encodings, they should set them in their own NixOS
configuration.