Fix quadratic build time when many infosets are emptied in game generators

[d-kad]

Fix quadratic build time during incremental tree construction

GameTreeRep::RemoveMember called RenumberInfosets unconditionally when
an infoset became empty, doing O(player.infosets.size()) work on every call.
I noticed this while profiling the subgame-roots checker on a depth-18 binary game
(~524K nodes), where build time dominated and ran into the minutes per game.

The current pattern is building a perfect-information tree and then merging siblings
into shared infosets via SetInfoset. Each merge empties a singleton infoset and triggers
the renumber: a linear sequence of merges becomes quadratic.

GameTreeRep already provides EnsureInfosetOrdering() as a lazy canonicalization,
and all public reads of GameInfosetRep::GetNumber should use it.

This PR:

• removes the eager RenumberInfosets from RemoveMember;

• makes GameInfosetRep::GetNumber() pass through EnsureInfosetOrdering
  explicitly, mirroring GameNodeRep::GetNumber().

For a 524K-node binary tree with sibling merging, 10 iterations of
build + is_subgame_root drop from 12 min 20 s to 1 min 45 s.
The speedup is entirely in the build phase; subgame detection itself is unaffected by this patch.