depends: Cleanup postprocess commands after switching to CMake

[hebasto]

I overlooked this while reviewing #29723, #29835, and #29880.

[DrahtBot]

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[hebasto]

My Guix build:

x86_64
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4a02c0d6e181303e9d88d18e51e1087f4db009dc0c326564e99aed2dcee2c96d  guix-build-f624854665ac/output/x86_64-w64-mingw32/bitcoin-f624854665ac-win64-debug.zip
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