Remove qDebug statement from registerCompressionLibraries()

This commit remove the debug statement originally introduced in 5f1616f.

Before this commit the text:

   Register compression libraries

was displayed each time the application instantiating the DICOM
database was started.
Do not display debug info when tagCacheExists method works as expected.

This commit removed debug statements originally added in 261841d0
while developing the class.
Merge branch 'remove-extra-debug-info'

* remove-extra-debug-info:
  Do not display debug info when tagCacheExists method works as expected.
  Remove qDebug statement from registerCompressionLibraries()