store_buffer: remove redundant fifo_raddr signal

This extra signal serves no purpose but to make the code harder
to read, connect read_pointer directly to raddr instead.
store_buffer: use re logic of ram instead of registering fifo_dout

Since the RAM already is keeping it's output data registered,
it's more area efficient to take advantage of this than
registering it in the store buffer.
Thanks to Florent Kermarrec for reminding me about this.