ipc mining: remove deprecated methods (incompatible schema change)

This change removes deprecated methods from the ipc mining interface.

Warning: This is an intermediate, review-only commit. Binaries built from it
should not be distributed or used to connect to other clients or servers. It
makes incompatible changes to the `mining.capnp` schema without updating the
`Init.makeMining` version, causing binaries to advertise support for a schema
they do not actually implement. Mixed versions may therefore exchange garbage
requests/responses instead of producing clear errors. The final commit in this
series bumps the mining interface number to ensure mismatches are detected.

git-bisect-skip: yes

src/interfaces/mining.h

@@ -42,31 +42,9 @@ public:
     // Sigop cost per transaction, not including coinbase transaction.
     virtual std::vector<int64_t> getTxSigops() = 0;
 
-    /**
-     * Return serialized dummy coinbase transaction.
-     *
-     * @note deprecated: use getCoinbaseTx()
-     */
-    virtual CTransactionRef getCoinbaseRawTx() = 0;
-
     /** Return fields needed to construct a coinbase transaction */
     virtual node::CoinbaseTx getCoinbaseTx() = 0;
 
-    /**
-     * Return scriptPubKey with SegWit OP_RETURN.
-     *
-     * @note deprecated: use getCoinbaseTx()
-     */
-    virtual std::vector<unsigned char> getCoinbaseCommitment() = 0;
-
-    /**
-     * Return which output in the dummy coinbase contains the SegWit OP_RETURN.
-     *
-     * @note deprecated. Scan outputs from getCoinbaseTx() outputs field for the
-     *       SegWit marker.
-     */
-    virtual int getWitnessCommitmentIndex() = 0;
-
     /**
      * Compute merkle path to the coinbase transaction
      *