This was introduced to allow creating a shared secret, but it's better to use
makesecret which creates unique secrets. getsharedsecret being a generic ECDH
function allows the caller to initiate conversations as if it was us; this
is generally OK, since we don't allow untrusted API access, but the commando
plugin had to blacklist this for read-only runes explicitly.
Since @ZmnSCPxj never ended up using this after introducing it, simply
remove it.
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
Changelog-Removed: JSONRPC: `getsharedsecret` API: use `makesecret`
9685c1adaf