Remove extraneous use of FLAG0 from tracks cleaner (it was checked but
never set).
Fix issue in expand connections where two parts of it couldn't agree
on the same flag (BUSY vs SKIP_STRUCT), and where the second part was
clearing the flag instead of setting it.
Remove obsolete HIGHLIGHT infrastructure (we now use selection).
Remove extraneous use of BUSY flag in several places (it was never
set).
Note: this is a basic implementation but it could be
improved once we include bus information in the netlist
and pcbnew can natively keep track of buses and nets
instead of just nets.
Fixes https://gitlab.com/kicad/code/kicad/-/issues/4158
Jeff Young
Jon Evans