fix(gatsby-plugin-netlify-cms): Don't use StaticQueryMapper (#30599)

The Netlify CMS plugin is spinning an extra webpack instance with almost
every plugin from gatsby. Reusing the `StaticQueryMapper` plugin caused
issues given how it's dealing with a Gatsby state.

Fixes #29951

This commit also:
- removes `cssTests` which wasn't used anymore
- adds a clear TODO for how to get `excludeAssets` to be taken into account again

packages/gatsby-plugin-netlify-cms/src/gatsby-node.js

@@ -28,9 +28,7 @@ function deepMap(obj, fn) {
  return obj
 }
 
-const cssTests = []
-
-function replaceRule(value, stage) {
+function replaceRule(value) {
  // If `value` does not have a `test` property, it isn't a rule object.
  if (!value || !value.test) {
  return value
@@ -96,20 +94,6 @@ exports.onCreateWebpackConfig = (
  return Promise.resolve()
  }
 
- // populate cssTests array later used by isCssRule
- if (`develop` === stage) {
- cssTests.push(
- ...[
- rules.cssModules().test,
- rules.css().test,
- /\.s(a|c)ss$/,
- /\.module\.s(a|c)ss$/,
- /\.less$/,
- /\.module\.less$/,
- ].map(t => t.toString())
- )
- }
-
  const gatsbyConfig = getConfig()
  const { program } = store.getState()
  const publicPathClean = trim(publicPath, `/`)
@@ -161,15 +145,19 @@ exports.onCreateWebpackConfig = (
  },
  module: {
  rules: deepMap(gatsbyConfig.module.rules, value =>
- replaceRule(value, stage)
+ replaceRule(value)
  ).filter(Boolean),
  },
  plugins: [
  // Remove plugins that either attempt to process the core Netlify CMS
  // application, or that we want to replace with our own instance.
  ...gatsbyConfig.plugins.filter(
  plugin =>
- ![`MiniCssExtractPlugin`, `GatsbyWebpackStatsExtractor`].find(
+ ![
+ `MiniCssExtractPlugin`,
+ `GatsbyWebpackStatsExtractor`,
+ `StaticQueryMapper`,
+ ].find(
  pluginName =>
  plugin.constructor && plugin.constructor.name === pluginName
  )
@@ -210,7 +198,8 @@ exports.onCreateWebpackConfig = (
  // Exclude CSS from index.html, as any imported styles are assumed to be
  // targeting the editor preview pane. Uses `excludeAssets` option from
  // `HtmlWebpackPlugin` config
- // not compatible with webpack 5
+ // HtmlWebpackExcludeAssetsPlugin is not compatible with webpack 5
+ // TODO: Replace `html-webpack-exclude-assets-plugin` with `html-webpack-skip-assets-plugin`
  // new HtmlWebpackExcludeAssetsPlugin(),
 
  // Pass in needed Gatsby config values.