Remove Windows support

Unfortunately, I don't have the expertise or bandwidth to maintain
the Windows support in Litestream. I'm open to re-adding support in
the future but right now it is hindering development and is not
well-tested or well-used.

internal/internal_unix.go

@@ -1,21 +0,0 @@
-// +build aix darwin dragonfly freebsd linux netbsd openbsd solaris
-
-package internal
-
-import (
-	"os"
-	"syscall"
-)
-
-// Fileinfo returns syscall fields from a FileInfo object.
-func Fileinfo(fi os.FileInfo) (uid, gid int) {
-	if fi == nil {
-		return -1, -1
-	}
-	stat := fi.Sys().(*syscall.Stat_t)
-	return int(stat.Uid), int(stat.Gid)
-}
-
-func fixRootDirectory(p string) string {
-	return p
-}