Delete apk's from the repo, use github releases.

It was a hasty decision to add all releases to the repo, it adds to the clone size and is just needed with github releases.
I won't do a rewrite because thay would break the countless forks clover had, it'll only be useful for clones with a low depth.

releases/README

@@ -0,0 +1 @@
+Clover APK releases signed by the author can be found at https://github.com/Floens/Clover/releases