nixos/networkmanager: fix merging options

Incorrect merging of modules resulted in dhcpcd being enabled causing flaky network connection.

https://github.com/NixOS/nixpkgs/pull/64364

Fixing it uncovered an infinite recursion from the same commit, previously masked by the incorrect merge.

We can just drop the `mkDefault` for `networking.wireless.enable` as it is already `false` by default.

Closes: https://github.com/NixOS/nixpkgs/issues/72416