build: remove unneeded rollup-plugin-sourcemaps dependency from root

The `rollup-plugin-sourcemaps` dependency was previously needed by the
`ng_package` rule but this has been moved to a separate legacy package.json
location within the repository and is no longer needed in the root package.json.

Author: clydin

Diff for: package.json

@@ -131,7 +131,6 @@
     "quicktype-core": "23.0.171",
     "rollup": "4.39.0",
     "rollup-license-plugin": "~3.0.1",
-    "rollup-plugin-sourcemaps": "^0.6.0",
     "semver": "7.7.1",
     "shelljs": "^0.9.0",
     "source-map-support": "0.5.21",