The engines section of package.json has traditionally been used to enforce a known good range of versions of node for local development. This has worked well so far for developers who work on the amphtml repo, where we require an active LTS version of node (and recommend the latest active LTS version of node).

However, we've had multiple instances in the past of the engines section of package.json coming in the way of developer workflows for external contributors who use sub-packages published from within the amphtml repo. See earlier relevant discussions in #19050, #19218, #25161, #25189, #25209, and #26408.

In addition, it has caused failures on Travis CI when node pushes out a new version, causing our package.json files to be outdated until they are manually updated.

When I spoke to @MylesBorins at Open JS last month, he mentioned to me that the engines section is likely to be deprecated soon, and that a better way of enforcing node versions in a repo is to have tests / CI return an error for incompatible versions. We already do this for amphtml via build-system/common/check-package-manager.js.

This PR deletes the engines section from all package.json files in amphtml.

Fixes #26408
Closes #25209
Related to #19050, #19218, #25161, #25189, #25209, and #26408.