removed extraneous error message from Upload lib on failure of validate_upload_path()

http://codeigniter.com/bug_tracker/bug/4390/
diff --git a/system/libraries/Upload.php b/system/libraries/Upload.php
index 0fc38dc..317cf95 100644
--- a/system/libraries/Upload.php
+++ b/system/libraries/Upload.php
@@ -144,7 +144,7 @@
 		// Is the upload path valid?

 		if (! $this->validate_upload_path())

 		{

-			$this->set_error('upload_no_filepath');

+			// errors will already be set by validate_upload_path() so just return FALSE

 			return FALSE;

 		}