Remove transitional GN code
Remove transitional GN code

Remove the global variable using_new_global_compiler_configs which is unnecessary now that all deps have been rolled to remove uses of it.

TBR=dpranke

Review URL: https://codereview.chromium.org/1349243002

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#349433}

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