Remove non-cpufreq_* modules since they are loaded by udev

In my system log:

    Jul 01 01:07:51 nixos systemd-modules-load[1376]: Failed to insert 'pcc_cpufreq': No such device
    Jul 01 02:41:31 nixos systemd-modules-load[1280]: Failed to insert 'pcc_cpufreq': No such device
    Jul 01 13:58:41 nixos systemd-modules-load[1280]: Failed to insert 'pcc_cpufreq': No such device
    Jul 05 21:55:34 nixos systemd-modules-load[29460]: Failed to insert 'pcc_cpufreq': No such device
    <etc.>
    Jul 14 01:48:45 nixos systemd-modules-load[1333]: Failed to insert 'pcc_cpufreq': No such device

According to [this page](https://wiki.archlinux.org/index.php/CPU_frequency_scaling#CPU_frequency_driver), the device-specific module is loaded on-demand as of kernel 3.4, through some udev-kernel interaction. This is indeed the case on my system. Supposing that other systems are similarly successful in auto-probing, there is no reason to specifically load the modules. According to [the source](http://lxr.free-electrons.com/source/drivers/cpufreq/cpufreq.c#L463) modules such as cpufreq_ondemand should also be loaded, but that doesn't seem to work so I'm leaving it in.