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/object-api-putobject_test.go

@@ -204,7 +204,7 @@ func testObjectAPIPutObjectDiskNotFound(obj ObjectLayer, instanceType string, di
 
 	// Take 8 disks down, one more we loose quorum on 16 disk node.
 	for _, disk := range disks[:7] {
-		removeAll(disk)
+		os.RemoveAll(disk)
 	}
 
 	testCases := []struct {
@@ -253,7 +253,7 @@ func testObjectAPIPutObjectDiskNotFound(obj ObjectLayer, instanceType string, di
 	}
 
 	// This causes quorum failure verify.
-	removeAll(disks[len(disks)-1])
+	os.RemoveAll(disks[len(disks)-1])
 
 	// Validate the last test.
 	testCase := struct {