Remove the MSVC 2010 and 2012 project files

Only the 2008 version is kept up to date at the moment.

The newer versions of MSVC can open and upgrade the 2008 version
files anyway, so by removing the unmaintained ones we reduce the
confusion and maintainance burden.

Additionally, the preferred way of building with MSVC is with the
makefile build system, according to the readme.