wallet: List all wallets in non-SQLite or non-BDB builds

This commit does not change behavior when bitcoin is built normally with both
the SQLite and BDB libraries. It just makes non-SQLite and non-BDB builds
more similar to the normal build. Specifically:

- It makes wallet directory lists always include all wallets so wallets don't
  appear missing depending on the build.

- It now triggers specific "Build does not support SQLite database format" and
  "Build does not support Berkeley DB database format" errors if a wallet can't
  be loaded instead of the more ambiguous and scary "Data is not in recognized
  format" error.

src/wallet/db.cpp

@@ -74,10 +74,6 @@ fs::path SQLiteDataFile(const fs::path& path)
 
 bool IsBDBFile(const fs::path& path)
 {
-#ifndef USE_BDB
-    return false;
-#endif
-
     if (!fs::exists(path)) return false;
 
     // A Berkeley DB Btree file has at least 4K.
@@ -103,10 +99,6 @@ bool IsBDBFile(const fs::path& path)
 
 bool IsSQLiteFile(const fs::path& path)
 {
-#ifndef USE_SQLITE
-    return false;
-#endif
-
     if (!fs::exists(path)) return false;
 
     // A SQLite Database file is at least 512 bytes.