[bug]: don't warn user about existing/old bbolt DB if they have selected SQLite and the .sqlite files exist

[ZZiigguurraatt]

After using lndinit migrate-db, when I startup lnd with --lnd.db.backend=sqlite --lnd.db.use-native-sql I get the following information in the log:

2025-04-14 05:32:01.152 [INF] LTND: Opening the main database, this might take a few minutes...
2025-04-14 05:32:01.229 [WRN] LTND: Found existing bbolt database file in /home/litd/.lnd/data/chain/bitcoin/regtest/wallet.db while using database type sqlite. Existing data will NOT be migrated to sqlite automatically!
2025-04-14 05:32:01.229 [WRN] LTND: Found existing bbolt database file in /home/litd/.lnd/data/graph/regtest/channel.db while using database type sqlite. Existing data will NOT be migrated to sqlite automatically!

This is a bit confusing and unnecessary information. I think it should only look for a bbolt DB and inform the user of its existence if the user has requested a SQLite DB AND the SQLite DB does not actually exist (likely because they never ran lndinit migrate-db yet). Also, if we do do these checks because the SQLite DB doesn't exist and we are trying to warn the user of what they actually have, we should also check for macaroons.db and sphinxreplay.db.

[Dhiren-Mhatre]

Just submitted a PR to fix this. I added checks so warnings only appear when there's no SQLite file yet. Should work for all the DB types mentioned in the issue. Let me know if you need any changes!