Remove deprecated SourceMapDevToolPlugin (can use Webpack's built-in one with external source maps)

Author: SteveSandersonMS

src/Microsoft.AspNetCore.SpaServices/npm/aspnet-webpack/src/SourceMapDevToolPlugin.ts

@@ -1,3 +1,2 @@
 export { createWebpackDevServer } from './WebpackDevMiddleware';
 export { loadViaWebpack } from './LoadViaWebpack';
-export { SourceMapDevToolPlugin } from './SourceMapDevToolPlugin';