contrib: remove pywinmake submodule

The submodule may need frequent bumps and should be managed
separately as a build tool by developers. This commit also adjusts
the CMakeLists.txt to restrict the generated build configurations
on Windows to only "Release" because as this is the only config
that is currently used and functional (PDBs are generated and other
debug mechanisms can be activated as needed).

Build instructions are added to the README.

Gitlab: #1027
Change-Id: I367a6ded1a53056bc862c37d8e64417415cc339f