CH11: remove section about segnet (segwit test network)

Main advantage of describing this historic feature was the idea that it
might be used again, but I think signets have fully replaced the future
usage.

ch09.asciidoc

@@ -616,20 +616,10 @@ In early 2017, testnet3 supports all the features of mainnet, including
 Segregated Witness (see <<segwit>>). Therefore, testnet3 can also be
 used to test Segregated Witness features.((("", startref="testnet09")))
 
-==== Segnet&#x2014;The Segregated Witness Testnet
 
 
-((("segnet")))In 2016, a special-purpose testnet was launched to aid in
-development and testing of Segregated Witness (aka segwit; see
-<<segwit>>). This test blockchain is called +segnet+ and can be joined
-by running a special version (branch) of Bitcoin Core.
 
-((("segwit (Segregated Witness)")))Since segwit was added to testnet3,
-it is no longer necessary to use segnet for testing of segwit features.
 
-In the future it is likely we will see other testnet blockchains that
-are specifically designed to test a single feature or major
-architectural change, like segnet.
 
 ==== Regtest&#x2014;The Local Blockchain