do not create /dev/rtc symlink, let systemd search for it if needed

The export of the RTCs hctosys flag is uneccesary, the kernel takes care
of the persistemt clock management itself, without any need for:
  CONFIG_RTC_HCTOSYS=y
  CONFIG_RTC_HCTOSYS_DEVICE="rtc0"

"Chaotic hardware platforms" without native kernel persistent clock
support will find the proper RTC with the logic rtc_open() without
the need for a custom symlink.