Remove redundant calls to keys() method of name_to_id

[imkasen]

Fix the problem that I mentioned in https://github.com/coqui-ai/TTS/issues/3434#issuecomment-1871181181.

name_to_id() in class SpeakerManager already returns dict_keys type, but keys() method is called again on name_to_id in speaker_names(), which results in the error AttributeError: 'dict_keys' object has no attribute 'keys'

[CLAassistant]

All committers have signed the CLA.

[stale[bot]]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions. You might also look our discussion channels.