Removed unexisting idgen.d from win32 makefile

[jercaianu]

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[andralex]

@RazvanN7 @WalterBright what's the deal with this? I'll pull because all tests pass.

[PetarKirov]

@andralex Since #6837 dmd identifier generation was transitioned from a build step to CTFE, so this PR is correct in removing idgen.d. I wonder however why the auto-tester didn't catch this in #6837...