[MC] Remove FK_Data_6b

D58335 introduced FK_Data_6b for emitting R_RISCV_SET6/R_RISCV_SUB6 in
.eh_frame/.debug_frame.

This is no longer needed after commit
c8ed138c removed unneeded fixup kinds
for R_RISCV_{SET,ADD,SUB}* and getKindForSizeInBits.