Revert "[WIN32] changed: use up to 4 parallel msbuild workers."

This reverts commit 162347f2b027ad7b333c5864a4153c5ece7c754f.

This change works fine if the windows vm is the only one compiling. Since the recent build logs showed a longer compile time when doing builds for all platforms I'll revert this for now to see if it was the culprit.