Remove obsolete others_in_slot plibs code.

Remove obsolete others_in_slot plibs code.

This code dates back to commit 79a2ade5a6cb5a6c2d7ace838a39a634265c5522
and the preserve-libs logic was different back then. At that time, the
dblink._preserve_libs() method would earlier register preserved libs
for the new package currently being merged, while the vardb entry was
still a -MERGING- node. Current code does not register preserve_paths
until later, so this others_in_slot code is obsolete.