sqlitepool: delete never-used code

This sqlitepool was a Jun 2022 effort to move away from Go's
database/sql package and have an SQLite-specific layer instead. But
we've never used it, or even finished it.

Instead, it just distracts people reading this repo, making them think
we use this code. So just delete it. We can always resurrect it from git
if we decide to go in that direction later.

Updates #cleanup

Signed-off-by: Brad Fitzpatrick <brad@danga.com>

Author: bradfitz