Revert plugin infrastructure

[balopat]

After lengthy discussions we decided to move away from the plugin model for builders and rather enable a lightweight extension model.

The cost of the a full-blown plugin model in terms of complexity, loss of functionality (schemas) and developer time would in the end benefit only a small fraction of potential and current skaffold users. We will support a selected few builders that are coming down the line, and most likely we won't take external contributions for most of the builders out there, only the ones that align well strategically with covering most of our targeted users.

An escape hatch - lightweight extension model is still going to be provided for users that are in need for specific image builders. The details of that are going to be worked out in the next couple of days.

This issue is to remove the hashicorp go-plugin related infrastructure. I will close all the plugin related issues as well referring to this decision.

[priyawadhwa]

Plan for reverting code:

• remove bazel plugin & revert to original structure because it's pretty self contained Remove bazel plugin & revert back to original #1989
• remove docker plugin & revert to original structure, which will remove almost all of the plugin code since GCB is tied to it as well Remove docker plugin and revert to original code structure #1990
• remove executionEnvironment and plugin fields from config, will require a config change Remove plugin code from config, upgrade to v1beta10 #2016
• make sure any additional cleanup is done (constants, documentation, integration test cases, etc)

[priyawadhwa]

This is done now, closing.