Simplifying Pixel schema

The PixelServicesTool package offers three different schema for the xml tags associated with pixel "simple services", of which two were never (or are no longer) used.

I've removed the class SimpleServiceVolumeSchema by deleting the redundant schema, and hard-coding the xml tags into PixelSimpleServiceXMLHelper (as done in other packages). After this, I found that the SimpleServiceVolumeMakerMgr class did nothing more than call similarly named functions in PixelSimpleServiceXMLHelper. I therefore refactored the code to remove SimpleServiceVolumeMakerMgr, and merge its private member variables into SimpleServiceVolumeMaker.

Comparisons against the output from the mass debugger tool have been a little tricky (the last nightly did not include the previous patch removing the volID). However, it appears that none of the volume's shapes, masses or dimensions have been altered by this set of changes.

Side note for future reference: InDetGeoModelUtils ​ServiceVolumeMaker.h/.cxx contains an apparently identical copy of SimpleServiceVolumeSchema and SimpleServiceVolumeMakerMgr. All of the changes done in this commit should probably be repeated in this other package.