fix(index): remove sourceMap warning (webpack#361)

Author: yyx990803

README.md

@@ -268,8 +268,6 @@ module.exports = ({ file, options, env }) => ({
 
 Enables source map support, `postcss-loader` will use the previous source map given by other loaders and update it accordingly, if no previous loader is applied before `postcss-loader`, the loader will generate a source map for you.
 
-> :warning: If a previous loader like e.g `sass-loader` is applied and it's `sourceMap` option is set, but the `sourceMap` option in `postcss-loader` is omitted, previous source maps will be discarded by `postcss-loader` **entirely**.
-
 **webpack.config.js**
 ```js
 {

lib/index.js

@@ -137,10 +137,6 @@ module.exports = function loader (css, map, meta) {
       css = this.exec(css, this.resource)
     }
 
-    if (!sourceMap && map) {
-      this.emitWarning(`\n\n ⚠️  PostCSS Loader\n\nPrevious source map found, but options.sourceMap isn't set.\nIn this case the loader will discard the source map entirely for performance reasons.\nSee https://github.com/postcss/postcss-loader#sourcemap for more information.\n\n`)
-    }
-
     if (sourceMap && typeof map === 'string') map = JSON.parse(map)
     if (sourceMap && map) options.map.prev = map