stakeKey registration stake.skey witness not needed

Since the start of Shelley we sign a stakeKey registration also with the stake.skey. Turnes out, this is wrong and not needed at all. 😱

According to https://hydra.iohk.io/build/6752481/download/1/delegation_design_spec.pdf
Chapter 3.3: "We do not require a witness to register a stake address (besides, of course, any witnesses needed for the transaction that is used to post the certificate)."

So only the certificate is needed and the witness for the payment of course.

Add an explanation for why we only need 1 witness

doc/stake-pool-operations/5_register_key.md

@@ -25,11 +25,16 @@ For the transaction draft, --tx.out, --invalid-hereafter and --fee can be set to
 
 #### Calculate fees
 
+Compute the minimum fee.
+
+Note that only one witness is required because
+[Chapter 3.3 "we do not require a witness to register a stake address"](https://github.com/input-output-hk/cardano-ledger/releases/latest/download/shelley-delegation.pdf).
+
     cardano-cli transaction calculate-min-fee \
     --tx-body-file tx.draft \
     --tx-in-count 1 \
     --tx-out-count 1 \
-    --witness-count 2 \
+    --witness-count 1 \
     --byron-witness-count 0 \
     --mainnet \
     --protocol-params-file protocol.json
@@ -79,7 +84,6 @@ Sign it:
     cardano-cli transaction sign \
     --tx-body-file tx.raw \
     --signing-key-file payment.skey \
-    --signing-key-file stake.skey \
     --mainnet \
     --out-file tx.signed