CheckTimeSync: Let "IgnoreService" truly ignore the local service

[RincewindsHat]

Previously the "-IgnoreService" flag did only prevent the specific IcingaCheck from becoming CRITICAL (which does not prevent it from becoming UNKNOWN!).
This does not really fit the definition of "ignore" in my book.

This commit changes the behaviour to completely ignore it, when the flag is set.

[cla-bot]

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[Lprod]

We have tested the solution in our company and the result meets the expectations of the “IgnoreService” function. A quick implementation and release would be nice from our point of view.
Thank you very much