Remove explicit sof-firmware as is already included

Previously, I'd added both the firmware and the `extraModprobeConfig`
that selects the DSP driver, however it seems only the latter is
necessary.

Addresses @terinjokes comment [here][1].

[1]: https://github.com/NixOS/nixos-hardware/pull/207/files#r534062750

dell/xps/13-9310/default.nix

@@ -11,9 +11,6 @@
   })];
 
   hardware.firmware = lib.mkBefore [
-    # Necessary for audio.
-    # https://bbs.archlinux.org/viewtopic.php?pid=1933548#p1933548
-    pkgs.sof-firmware
     # Firmware for the AX500 (wi-fi & bluetooth chip).
     pkgs.qca6390-firmware
   ];