remove multibuild submodule and appveyor.yml

We now use Github CI and cibuildwheel for all three OSes

Author: anthrotype

.gitmodules

@@ -1,6 +1,3 @@
-[submodule "multibuild"]
-	path = multibuild
-	url = https://github.com/matthew-brett/multibuild/
 [submodule "src/cpp/skia-builder"]
 	path = src/cpp/skia-builder
 	url = https://github.com/fonttools/skia-builder