Remove IE8 Gulp compile tasks

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What

Update tasks/gulp/compile-assets.js to remove the IE8-specific compilation tasks. There will likely be side-effects (for example: the update-assets-version task will fail because IE8 files are not compiled).

Why

We have made the decision to move away from Gulp and write our own build scripts. Additionally, we will be dropping support for IE8. A good deal of dependency management is reliant on us removing oldie: #2469

Who needs to work on this

Developers

Who needs to review this

Developers

Done when

• IE8 tasks in tasks/gulp/compile-assets.js have been removed
• Tasks no longer depend on taskArguments.destination
• docs/contributing/tasks.md has been updated
• Dependencies have been updated or removed

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I'm going to close this as we've dealt with the gulp aspect of these IE8 compile tasks: #2469

The actual removal of the IE8 compile tasks can be part of the work to remove IE8 in general.