Remove mUserEpollWakeup from EventHub

All devices supporting Oreo must have a linux kernel version 3.18 or
above, meaning it isn't necessary to check if the kernel version is above
3.5.

Bug: 118001226
Test: After compiling and creating a build and installing on device,
there were no crashes.

Change-Id: I74f04524199b9220d836c91f8e71e85fe75ba173