Remove some unused variables triggering "unused variable" warnings.

Signed-off-by: Christian Stewart <christian@paral.in>

Adds a new option "useAbortSignal" which adds an optional AbortSignal parameter
to RPC functions. AbortController and AbortSignal are built-ins in both Node.JS
and all web browsers, which implement aborting long-lived processes.

For example:

const abortController = new AbortController()
const responsePromise = rpcClient.DoSomething(request, abortController.signal)
// abort the RPC call early
abortController.abort()

Fixes stephenh#730

Signed-off-by: Christian Stewart <christian@paral.in>