Use webpack instead of rollup for the legacy build

This also changes the target path of the legacy build to build/legacy

package.json

@@ -18,7 +18,7 @@
     "build-examples": "webpack --config examples/webpack/config.js --mode production",
     "build-package": "npm run transpile && npm run copy-css && node tasks/prepare-package && cp README.md build/ol",
     "build-index": "npm run build-package && node tasks/generate-index",
-    "build-legacy": "rm -rf build && npm run build-index && rollup --config config/rollup.js && cleancss --source-map src/ol/ol.css -o build/ol.css",
+    "build-legacy": "rm -rf build && npm run build-index && webpack --config config/webpack-config-legacy-build.js && cleancss --source-map src/ol/ol.css -o build/legacy/ol.css",
     "copy-css": "cp src/ol/ol.css build/ol/ol.css",
     "transpile": "rm -rf build/ol && mkdir -p build && buble --input src/ol --output build/ol --no modules --sourcemap",
     "typecheck": "tsc --pretty",
@@ -74,11 +74,6 @@
     "pixelmatch": "^4.0.2",
     "proj4": "2.5.0",
     "rollup": "0.66.6",
-    "rollup-plugin-buble": "0.19.4",
-    "rollup-plugin-commonjs": "9.2.0",
-    "rollup-plugin-node-resolve": "3.4.0",
-    "rollup-plugin-sourcemaps": "0.4.2",
-    "rollup-plugin-uglify": "6.0.0",
     "sinon": "^6.0.0",
     "typescript": "^3.1.0-dev.20180905",
     "uglifyjs-webpack-plugin": "^2.0.1",