[openload] Remove a confusing exception

[openload] Remove a confusing exception

If phantomjs is not installed, there's an error besides the missing
phantomjs exception:

Exception ignored in: <bound method PhantomJSwrapper.__del__ of <youtube_dl.extractor.openload.PhantomJSwrapper object at 0x7f8ad5e78278>>
Traceback (most recent call last):
  File "/home/yen/Projects/youtube-dl/youtube_dl/extractor/openload.py", line 142, in __del__
    os.remove(self._TMP_FILES[name].name)
AttributeError: 'PhantomJSwrapper' object has no attribute '_TMP_FILES'