CH04::dumpprivkey: remove

Remove text about dumping a private key:

- Example using Bitcoin Core is no longer supported for descriptor
  wallets.

- Dumping private keys is very bad practice with HD wallets due to risk
  of compromising the whole wallet.

- Because of safety risks, and lack of need, most modern wallets don't
  support private key export or import

ch04.asciidoc

@@ -182,45 +182,6 @@ large number. It is approximately 10^77^ in decimal. For comparison, the
 visible universe is estimated to contain 10^80^ atoms.
 ====
 
-((("dumpprivkey command")))To generate a new key with the Bitcoin Core
-client (see <<ch03_bitcoin_client>>), use the +getnewaddress+ command.
-For security reasons it displays the public key only, not the private
-key. To ask +bitcoind+ to expose the private key, use the +dumpprivkey+
-command. The +dumpprivkey+ command shows the private key in a Base58
-checksum-encoded format called the _Wallet Import Format_ (WIF), which
-we will examine in more detail in <<priv_formats>>. Here's an example of
-generating and displaying a private key using these two commands:
-
-----
-$ bitcoin-cli getnewaddress
-1J7mdg5rbQyUHENYdx39WVWK7fsLpEoXZy
-$ bitcoin-cli dumpprivkey 1J7mdg5rbQyUHENYdx39WVWK7fsLpEoXZy
-KxFC1jmwwCoACiCAWZ3eXa96mBM6tb3TYzGmf6YwgdGWZgawvrtJ
-----
-
-The +dumpprivkey+ command opens the wallet and extracts the private key
-that was generated by the +getnewaddress+ command. It is not possible
-for +bitcoind+ to know the private key from the public key unless they
-are both stored in the wallet.
-
-[TIP]
-=====================================================================
-The +dumpprivkey+ command does not generate a private key from a public
-key, as this is impossible. The command simply reveals the private key
-that is already known to the wallet and which was generated by the
-+getnewaddress+ command.
-=====================================================================
-
-[role="pagebreak-before"]
-You can also use the Bitcoin Explorer command-line tool (see
-<<appdx_bx>>) to generate and display private keys with the commands
-+seed+, +ec-new+, and +ec-to-wif+:
-
-----
-$ bx seed | bx ec-new | bx ec-to-wif
-5J3mBbAH58CpQ3Y5RNJpUKPE62SQ5tfcvU2JpbnkeyhfsYB1Jcn
-----
-
 [[pubkey]]
 ==== Public Keys