Another max_piece_type() cleanup.

Since this is already getting the pedantic treatment here is my submission.

Timings using AMD Phenom II, gcc 4.9.1, build ARCH=x86-64-modern
start /B /REALTIME /AFFINITY 0x1 stockfish.exe bench 1>nul

Master
4134
4134
4134
4134
4134  (yes I really did get exactly 4134 5 times in a row)

Patch
4119
4119
4118
4118
4118

Very minor speedup but it also removes a line.