Consolidate pawn storm types

Simplification: the Unopposed and Unblocked pawn storm types are mathematically
similar enough to combine with no Elo loss. This reduces the pawn storm types
to BlockedByPawn and UnBlocked.

STC
LLR: 2.95 (-2.94,2.94) [-3.00,1.00]
Total: 116869 W: 23549 L: 23605 D: 69715
http://tests.stockfishchess.org/tests/view/5af2def90ebc5968e6523f82

LTC
LLR: 2.95 (-2.94,2.94) [-3.00,1.00]
Total: 39912 W: 6090 L: 5998 D: 27824
http://tests.stockfishchess.org/tests/view/5af3b2e20ebc5968e6524013

Closes https://github.com/official-stockfish/Stockfish/pull/1592

Bench: 5244314