Delete the mk folder

This commit deletes the old build system located in the `mk` folder as it's now been obsoleted for two cycles and is replaced by rustbuild.
rust-lang · bors · Feb 8, 2017 · Jan 23, 2017 · Jan 23, 2017 · Jan 23, 2017
commit 9ab8090bd4c8fed848cfebe2ab818a91e43e225c
diff --git a/Makefile.in b/Makefile.in
diff --git a/mk/cfg/aarch64-apple-ios.mk b/mk/cfg/aarch64-apple-ios.mk
diff --git a/mk/cfg/aarch64-linux-android.mk b/mk/cfg/aarch64-linux-android.mk
diff --git a/mk/cfg/aarch64-unknown-fuchsia.mk b/mk/cfg/aarch64-unknown-fuchsia.mk
diff --git a/mk/cfg/aarch64-unknown-linux-gnu.mk b/mk/cfg/aarch64-unknown-linux-gnu.mk