Reduce minimum memory requirement by 16MB

Stockfish allocates the default hash (32MB) in main(),
before entering UCI::loop(). If there is not enough memory,
the program will crash even before UCI::loop() is entered
and the GUI is given a change to specify a lower Hash value.

So leave the current design, but reduce the default hash to 16MB
instread of 32MB. In order to perform such quick searches (depth=13),
there is no reason to use so much Hash anyway. Another benefit is
to introduce a bit of hash pressure in bench, which increases chances
to detect rare bugs related to TT replacement, for example.

This is not a functional change, although it obviously changes the bench.

bench 8072446
Remove insufficient material rule

The eval already returns zero in KK, KBK, KNK (see material.cpp).
The difference is:

- we lose the "TB pruning" benefit of the
  draw rule (ie. search goes on even if eval is zero)

- we gain some speed by removing a useless test from the hot path

Tested in no-regression mode because is a simplification.

Stopped at 128K games at STC:
LLR: 0.05 (-2.94,2.94) [-3.00,1.00]
Total: 128000 W: 21357 L: 21560 D: 85083

But passed LTC:
LLR: 2.96 (-2.94,2.94) [-3.00,1.00]
Total: 33023 W: 4613 L: 4509 D: 23901

bench 8070261