[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