Remove redundant custom chunk preloading code

[rtsao]

This code was a workaround to allow for async chunks to be preloaded (without being fetched twice) with webpack 3 (Fusion was originally implemented before webpack 4 was released).

The need for this workaround is no longer needed as it has been fixed in webpack 4 (this is the relevant PR).

[KevinGrandon]

Do we still have access to this information? Pretty soon we're going to be implementing service workers which will need to know all of the "initial", and non-dynamic scripts for a page so we can cache them for a second load.

[rtsao]

Yep, the information gathering part is still all there, this plugin was just for making the webpack 3 runtime able to see if some async chunks were already preloaded (which isn't needed anymore since this works out of the box in webpack 4)

[KevinGrandon]

LGTM then, happy to review once tests are passing.

[rtsao]

!merge