[lib] Get rid of SentencePrefixSearchIndex.entries

Summary: With PrefixTree we no longer need to special-case this.

Test Plan: I went to a community in my local environment and typed a single "@" character. I made sure the options displayed before/after were the same

Reviewers: tomek, atul, inka, rohan

Reviewed By: tomek

Subscribers: wyilio

Differential Revision: https://phab.comm.dev/D9646