[RISCV] Remove unneeded fixup kinds for R_RISCV_{SET,ADD,SUB}*

These 8/16/32/64 fixup kinds from D103539 are no longer needed after
D155357.

R_RISCV_SET6/R_RISCV_SUB6 are unneeded even before D155357.
`return Value & 0x03` is incorrect, and the non-zero TargetOffset is
probably to affect applyFixup, but the relevant code is dead as Value == 0.