Remove scp target from win32.mak

[marler8997]

Instead of moving the scp target to build.d, @wilzbach indicates (#10606 (review)) it is no longer used so can be removed.

This is one of the last targets in win32.mak holding a reference to the source file lists in it. Once we finish removing these last targets, we can remove the redundant source lists.

[dlang-bot]

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Testing this PR locally

If you don't have a local development environment setup, you can use Digger to test this PR:

dub fetch digger
dub run digger -- build "master + dmd#10608"

[MoonlightSentinel]

Remove scp from the targets list