[TTS] Remove unused TTS eval function
Signed-off-by: Jocelyn Huang <jocelynh@nvidia.com>

# What does this PR do ?

Removes `eval_tts_scores()`, which was unused and had `pesq` (unmaintained) and `pystoi` dependencies.

**Collection**: TTS

# Changelog 
- Removed `pesq` and `pystoi` install requirements for TTS
- Removed unused function from `helpers.py`
