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/erasure_test.go

@@ -18,6 +18,7 @@ package cmd
 
 import (
 	"bytes"
+	"os"
 	"testing"
 )
 
@@ -189,7 +190,7 @@ type erasureTestSetup struct {
 // Removes the temporary disk directories.
 func (e erasureTestSetup) Remove() {
 	for _, path := range e.diskPaths {
-		removeAll(path)
+		os.RemoveAll(path)
 	}
 }