Get rid of libwinpthread when switching tor to mingw-w64-clang

There is no need to ship libwinprthread when compiling tor with mingw-w64-clang. See: legacy/trac#31584 (moved) for context.

Note this is strictly speaking a parent ticket for legacy/trac#29321 (moved). However, Trac thinks any child can only have one parent. So, here we are.