Remove stale duplicate nucleitpcpbpb.cxx index entry

The rename to nucleitpcPbPb.cxx in d5e065d (#17177) was done from a
case-insensitive filesystem, so the old lowercase path never got
removed from the tree. Both paths collide into one file on
case-insensitive filesystems (macOS/Windows), making the tracked file
unrestorable via reset/checkout since git pathspec matching is
case-insensitive here.

Author: mpuccio