Protean piece attribute is redundant

[dturevski]

As currently implemented in solving/machinery/solvers.c:

if (CondFlag[protean] || TSTFLAG(some_pieces_flags,Protean))
    solving_insert_protean_chess(si);

it is impossible to mix Protean and non-Protean pieces as the effect of having a single Protean piece is the same as in setting Condition ProteanChess. It makes this attribute redundant and confusing.