main-cleanup-PixelDetectorManager

Addressing ATLASRECTS-4886. This is a >5 year old ticket due to warnings in the output, which apparently has never been commented on by the pixel software experts. In fact the warnings are generated at the level of the base class, InDetDetectorManager. There is already a mechanism in place to reduce repeated warnings (which are only due to the pixel detector). I decided to take advantage of this mechanism to completely suppress the warnings but only for the derived class PixelDetectorManager.