client: remove unused indirection for referencing consul executable

Was thinking about using the testing pattern where you create executable
shell scripts as test resources which "mock" the process a bit of code
is meant to fork+exec. Turns out that wasn't really necessary in this case.

client/allocrunner/taskrunner/envoybootstrap_hook.go

@@ -43,9 +43,6 @@ type envoyBootstrapHook struct {
 	// Consul's gRPC endpoint.
 	consulHTTPAddr string
 
-	// executable is executable file that is consul
-	executable string
-
 	// logger is used to log things
 	logger hclog.Logger
 }