Delete empty file

[irevoire]

Hello, I noticed the files:

• milli/src/heed_codec/facet/facet_string_level_zero_value_codec.rs
• milli/src/heed_codec/facet/facet_string_zero_bounds_value_codec.rs

Are empty and should be deleted.

[thenishantsapkota]

I would like to do this!