Revert "[csrng] Temporarily hack-in disable handling"

The reverted commit was originally introduced as part of
lowRISC#14410 to work around a temporary issue until
lowRISC#14534 got merged. It seems we forgot to remove that
temporary workaround before merging the PR.

The workaround is critical because it weakens a security countermeasure
inside CSRNG: errors injected into the command stage FSMs are no longer
fatal. By disabling and re-enabling the module these FSMs can be made to
leave the "terminal" error state.

For this reason, we need to remove this workaround. This reverts commit
f4df12f.

Signed-off-by: Pirmin Vogel <[email protected]>

hw/ip/csrng/rtl/csrng_cmd_stage.sv

@@ -401,12 +401,6 @@ module csrng_cmd_stage import csrng_pkg::*; #(
  cmd_stage_sm_err_o = 1'b1;
  end
  endcase // unique case (state_q)
-
- // if disable, cycle back to Idle
- if (!cs_enable_i && state_q != Idle) begin
- state_d = Idle;
- end
-
  end
 
  //---------------------------------------------------------