Remove explicit json version

The `client` crate uses a path dependency for `bitcoincore-rpc-json`,
there is no need to explicitly mention the version, doing so just makes
more maintenance work.

Author: tcharding

client/Cargo.toml

@@ -22,7 +22,7 @@ path = "src/lib.rs"
 rand = ["bitcoincore-rpc-json/rand"]
 
 [dependencies]
-bitcoincore-rpc-json = { version = "0.18.0", path = "../json" }
+bitcoincore-rpc-json = { path = "../json" }
 
 log = "0.4.5"
 jsonrpc = { version = "0.18.0", features = ["minreq_http"] }