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_windows.go

@@ -1,22 +0,0 @@
-// +build windows
-
-package internal
-
-import (
-	"os"
-)
-
-// Fileinfo returns syscall fields from a FileInfo object.
-func Fileinfo(fi os.FileInfo) (uid, gid int) {
-	return -1, -1
-}
-
-// fixRootDirectory is copied from the standard library for use with mkdirAll()
-func fixRootDirectory(p string) string {
-	if len(p) == len(`\\?\c:`) {
-		if os.IsPathSeparator(p[0]) && os.IsPathSeparator(p[1]) && p[2] == '?' && os.IsPathSeparator(p[3]) && p[5] == ':' {
-			return p + `\`
-		}
-	}
-	return p
-}