Drop unused columns

We are currently using libsqlite3-sys 0.25.2,
corresponding to SQLcipher 4.5.2
and SQLite 3.39.2.

SQLite supports ALTER TABLE DROP COLUMN since version 3.35.0,
and it has received critical database corruption bugfixes in 3.35.5.
There have been no fixes to it between SQLite 3.36.0 and 3.41.0,
so it appears stable now.

CHANGELOG.md

@@ -8,6 +8,7 @@
 - Run `cargo-deny` in CI. #4101
 - Check provider database with CI. #4099 
 - Switch to DEFERRED transactions #4100
+- Drop unused SQL columns #4141
 
 ### Fixes
 - Do not block async task executor while decrypting the messages. #4079