When migrating zone.js from gulp to bazel, some legacy build config files are still there,
we have `rollup-es5.config.js` and `rollup-es5_global-es2015.config.js`, since in gulp build
system, build `es5` or `esm` files are set in the config file, but in the bazel world,
the output format is not config in the config.js file, but is required by the downstream
bazel target. So we don't really need the two rollup config files any longer.
Another difference is in `rollup-es5.config.js`, the `external` and `global` libraries names
are also config there, and these settings are also valid for `es2015` build, these settings
are not in the `es2015.config.js` for some legacy reasons. So we don't need to keep this
difference either.
PR Close#40481
Daniel Díaz
Joey Perrott
JiaLiPassion