fpu_valid_opcode() did not correctly handle 16 bit fp instructions

fpu_valid_opcode() did not correctly handle 16 bit fp instructions
such as an stval of 0xaa22.  The RISC-V Instruction Set Manual states
that setting stval to a non-zero value with the instruction on illegal
instruction exception is an optional feature so instead of changing
fpu_valid_opcode() remove it entirely.

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