Use rml[0].pv[] instead of dedicated pv[] array

We have a small functionality change in case we have a
fail-high so that both rml[].pv and pv[] are updated, but if,
after researching, we have a fail-low then rml score is updated
again but pv[] remains the same and coming back from search we
used a PV line that has failed-low (after having failed-high).

With this patch we always use the 'correct' PV line, i.e. the
line with highest score at the end of the whole search.

Retire also redundant RootMove's 'move' member and directly
use pv[0] instead.