util.sqlite3: Don't cache prepared statements for one-off queries

The :execute method is mainly used for one-off queries such as creating
tables and indices. There is no need to cache this prepared statement,
as those queries are only done on startup.

Further, prepared statements can't be reused without being reset, so
this was likely broken anyway.

util/sqlite3.lua

@@ -163,6 +163,7 @@ end
 function engine:ondisconnect() -- luacheck: ignore 212/self
 	-- Override from create_engine()
 end
+
 function engine:execute(sql, ...)
 	local success, err = self:connect();
 	if not success then return success, err; end
@@ -178,20 +179,17 @@ function engine:execute(sql, ...)
 		return true;
 	end
 
-	local stmt = prepared[sql];
-	if not stmt then
-		local err;
-		stmt, err = self.conn:prepare(sql);
+	local stmt, err = self.conn:prepare(sql);
 	if not stmt then
 		err = sqlite_errors.new(err);
 		err.text = self.conn:errmsg();
 		return stmt, err;
 	end
-		prepared[sql] = stmt;
-	end
 
 	local ret = stmt:bind_values(...);
-	if ret ~= lsqlite3.OK then return nil, sqlite_errors.new(ret, { message = self.conn:errmsg() }); end
+	if ret ~= lsqlite3.OK then
+		return nil, sqlite_errors.new(ret, { message = self.conn:errmsg() });
+	end
 	return stmt;
 end