[lldb] Don't read firstSubclass and nextSiblingClass from class_rw_t (#171213)
We're considering modifying the ObjC runtime's class_rw_t structure to
remove the firstSubclass and nextSiblingClass fields in some cases. LLDB
is currently reading those but not actually using them. Stop doing that
to avoid issues if they are removed by the runtime.
rdar://166084122