cappuccino/rf: combine 'last_wb_result_b' and 'wb_to_decode_result'

Those two conditions are exclusive to each other and
can because of that share the register where the result is
stored.
cappuccino/execute_ctrl: remove unused ctrl_op_jal_o signal
cappuccino/rf: remove unnecessary decode to decode bypass

As the comment states, the bypass is only active when
there is such a condition that a 'l.jr r9' is in the
delay slot of a 'l.jal(r)' and since placing
a jump instruction in a delay slot causes undefined behaviour,
there's no need to handle that case.