posix: Deprecate custom removeAll/mkdirAll implementations. (#4808)

Since go1.8 os.RemoveAll and os.MkdirAll both support long
path names i.e UNC path on windows. The code we are carrying
was directly borrowed from `pkg/os` package and doesn't need
to be in our repo anymore. As a side affect this also
addresses our codecoverage issue.

Refer #4658

cmd/posix-utils_nix.go

@@ -31,20 +31,3 @@ func osStat(name string) (os.FileInfo, error) {
 func isValidVolname(volname string) bool {
 	return !(len(volname) < 3 || len(volname) > 63)
 }
-
-// mkdirAll creates a directory named path,
-// along with any necessary parents, and returns nil,
-// or else returns an error. The permission bits perm are used
-// for all directories that mkdirAll creates. If path is already
-// a directory, mkdirAll does nothing and returns nil.
-func mkdirAll(path string, perm os.FileMode) error {
-	return os.MkdirAll(path, perm)
-}
-
-// removeAll removes path and any children it contains.
-// It removes everything it can but returns the first error
-// it encounters.  If the path does not exist, RemoveAll
-// returns nil (no error).
-func removeAll(path string) error {
-	return os.RemoveAll(path)
-}