Revert "linux: disable UEVENT_HELPER*"

This reverts commit 9f87f3ccb0 because
it causes /proc/sys/kernel/hotplug to not be cleared on Linux <= 3.14.

nixos/modules/services/hardware/udev.nix

@@ -236,6 +236,8 @@ in
 
     system.activationScripts.udevd =
       ''
+        echo "" > /proc/sys/kernel/hotplug
+
         # Regenerate the hardware database /var/lib/udev/hwdb.bin
         # whenever systemd changes.
         if [ ! -e /var/lib/udev/prev-systemd -o "$(readlink /var/lib/udev/prev-systemd)" != ${config.systemd.package} ]; then

pkgs/os-specific/linux/kernel/common-config.nix

@@ -269,9 +269,6 @@ with stdenv.lib;
   SLIP_COMPRESSED y # CSLIP compressed headers
   SLIP_SMART y
   THERMAL_HWMON y # Hardware monitoring support
-  ${optionalString (versionAtLeast version "3.15") ''
-    UEVENT_HELPER n
-  ''}
   USB_DEBUG? n
   USB_EHCI_ROOT_HUB_TT y # Root Hub Transaction Translators
   USB_EHCI_TT_NEWSCHED y # Improved transaction translator scheduling