clockevents: remove WARN_ON which was used to gather information

The issue of the endless reprogramming loop due to a too small
min_delta_ns was fixed with the previous updates of the clock events
code, but we had no information about the spread of this problem. I
added a WARN_ON to get automated information via kerneloops.org and to
get some direct reports, which allowed me to analyse the affected
machines.

The WARN_ON has served its purpose and would be annoying for a release
kernel. Remove it and just keep the information about the increase of
the min_delta_ns value.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>