lnwallet+kvdb: remove duplicate bdb backend imports

We only want to register the bbolt DB backend ("bdb") when we're not
compiling for a JS/WASM build targets. That's why we want to have that
import in only one file that we can add a build tag to. We remove it in
two other places since only one import is enough anyway.

kvdb/interface.go

@@ -2,7 +2,6 @@ package kvdb
 
 import (
 	"github.com/btcsuite/btcwallet/walletdb"
-	_ "github.com/btcsuite/btcwallet/walletdb/bdb" // Import to register backend.
 )
 
 // Update opens a database read/write transaction and executes the function f