Remove warmup workaround

[juanjoDiaz]

This PR removes the workaround to the issues with the wamup plugin.

That workaround only worked with versions 4.X of warmup since version 5.X change the folder from _warmup to .warmup.
Also, I modified the warmup plugin use of lifecycle hooks so it will not conflict with this or the webpack plugin as long as it's declared after it.

[jayair]

Thanks @juanjoDiaz! Appreciate the PR!

Would keeping this fix around make sense if somebody is still using 4.x? And, does the fix break for somebody using 5.x?

[juanjoDiaz]

This workaround still works for v4.x (as long as they don't change the default setting that created the _warmup folder).
This doesn't break anything for the rest of the users as long as they don't have a folder named _warmup.

This workaround doesn't work in v5 because v5 doesn't use _warmup, it uses custom names that are given by the user like .warmup/<my_warmer>/index and there can be multiple entries.

And it's not even needed after v5.2.1 since I added a workaround on my end.

To be honest, people should update to v5. The upgrade is trivial and there is nothing in v4 that's not in v5.

[thomaschaaf]

implemented in #208