plugin: remove unnecessary import "C" from example

It seems that it is not needed to import the pseudo package "C"
for the plugin to be built correctly.
Removing it to avoid confusion.

Change-Id: I62838a953ad2889881bfbfd1a36141661565f033
Reviewed-on: https://go-review.googlesource.com/36638Reviewed-by: David Crawshaw <crawshaw@golang.org>

src/plugin/plugin.go

@@ -44,9 +44,6 @@ func (p *Plugin) Lookup(symName string) (Symbol, error) {
 //
 //	package main
 //
-//	// // No C code needed.
-//	import "C"
-//
 //	import "fmt"
 //
 //	var V int