Remove -t msvc

Its functionality is (almost entirely) subsumed by deps=msvc. The only thing that it's used for is to help gyp. Gyp needs to set the environment that the compiler is run in, and this was stuffed into -t msvc out of convenience long ago.

At this point -t msvc is just confusing, and should be removed, with gyp fending for itself in some other way (perhaps by adding an env_block variable to ninja).
