Remove gulpfile from release

[hpinkos]

Fixes #3911

The release zips are only meant for using Cesium, not building it, so there's no need to include the gulpfile.

[cesium-concierge]

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[pjcozzi]

Anyone else want to look at this before we merge?

[mramato]

I'm not sure this really fixes the issue because there will still be gulp-based tasks listed in package.json, so it's not going to stop our users from attempting to run build tasks against the release zip and submitting github issues or forum posts because they think it's broken. We could modify the package.json during zip creation so that it only include support commands.

But the real question we need to answer is, "What else are do we support doing with the release zip?":

1. Should you be able to rebuild Cesium (and Apps and documentation)?
2. Should you be able to run all tests?
3. Should you be able to run jsHint/eslint and other build stuff?

Depending on the answers to these questions (and maybe others I glanced over) we can modify the zip further to better suit the needs.

[pjcozzi]

2. yes

3. and 3. sure, if it is easy enough.

No rush on this, just curating some PRs for 1.41.

[hpinkos]

Closing this since it doesn't seem like the solution we want. Continue discussion in #3911