Reverts "Expose build mode in environment of asset transformer processes (#144752)"

[auto-submit]

Reverts: #144752

Initiated by: andrewkolos

Reason for reverting: compilation issue has turned the tree red

Original PR Author: andrewkolos

Reviewed By: {christopherfujino}

This change reverts the following previous change:
In service of #143348

When invoking a package to transform an asset, we set FLUTTER_BUILD_MODE to the CLI name of the build mode being used. Inspired by #101077 (comment):

Do transformers know whether they get executed in debug or release mode? I kinda imagine that being useful. Ex: There's a transformer that optimizes the file size of images. Depending on the amount and size of the images, that could take a significant amount of time. Therefore, I might want to only execute it in release builds.

Note for the reviewer: the interesting part of this change can be found in the commit set environment variable to build mode when running asset transformer…. The rest of the change is updating call sites with a new argument.

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