Utilities/Release: Drop docker specs for Windows binaries

Since commit 4c7c66dcf5 (gitlab-ci: Add jobs to make Windows x86_64 and
i386 packages, 2022-05-19), we use CI jobs to produce Windows binaries.
Revert commit ff929badb3 (Utilities/Release: Add docker specs to
build and test Windows binaries, 2020-05-05, v3.18.0-rc1~203^2~1).
The filesystem inside Windows containers is too slow to support
parallel builds efficiently.