Remove istanbul JavaScript package

Signed-off-by: Takuya Noguchi <takninnovationresearch@gmail.com>

changelogs/unreleased/63971-remove-istanbul.yml

+---
+title: Remove istanbul JavaScript package
+merge_request: 30232
+author: Takuya Noguchi
+type: other

package.json

@@ -168,7 +168,6 @@
     "gettext-extractor": "^3.4.3",
     "gettext-extractor-vue": "^4.0.2",
     "graphql-tag": "^2.10.0",
-    "istanbul": "^0.4.5",
     "jasmine-core": "^2.9.0",
     "jasmine-diff": "^0.1.3",
     "jasmine-jquery": "^2.1.1",