Fix some 's' cases for IPA G2P
Signed-off-by: Jocelyn Huang <jocelynh@nvidia.com>

# What does this PR do ?

Fix some handling of "s" cases in the IPA G2P class.

**Collection**: TTS

# Changelog 
- Indexing fixed for cases where there's no apostrophe
- Removed extraneous "t" added for "ts/t's"
- Removed "s" case when there's no apostrophe
  
**PR Type**:
- [ ] New Feature
- [x] Bugfix
- [ ] Documentation