Revert "mk: fix some undefined variable warnings"

[alexcrichton]

This reverts commit d037129.

[rust-highfive]

r? @nikomatsakis

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[alexcrichton]

This reverts #30367 which looks like it unfortunately caused a 1hr long regression in testing time on the bots (compare this short build to this long build).

I may try to investigate later, but seems good to at least get the cycle time back down for now.

cc @tamird

r? @brson

[tamird]

:(

[brson]

@bors r+ p=1

[bors]

📌 Commit 178d4b0 has been approved by brson

[Manishearth]

@bors p=20

[bors]

☀️ Test successful - auto-linux-32-nopt-t, auto-linux-32-opt, auto-linux-64-debug-opt, auto-linux-64-nopt-t, auto-linux-64-opt, auto-linux-64-x-android-t, auto-linux-cross-opt, auto-linux-musl-64-opt, auto-mac-32-opt, auto-mac-64-nopt-t, auto-mac-64-opt, auto-mac-ios-opt, auto-win-gnu-32-nopt-t, auto-win-gnu-32-opt, auto-win-gnu-64-nopt-t, auto-win-gnu-64-opt, auto-win-msvc-32-opt, auto-win-msvc-64-opt