Cleaning up by replacing occurrences of "is_blayer" with the newly implemented "is_innermost".
Follow up on changes done for fixing the ITk identifier scheme.
Summary from the old MR: Redefines b-layer for SLHC layouts (see below). PixelID::is_blayer(id) becomes obsolete and will be replaced with PixelID::is_innermost(id). A warning is thrown in order to identify clients: !23552 (merged)
Further discussion on this: https://its.cern.ch/jira/browse/ATLITKSW-72