lmdb: fix gcc 7.3.0 'implicit-fallthrough' warning

src/blockchain_db/lmdb/db_lmdb.cpp

@@ -4358,16 +4358,12 @@ void BlockchainLMDB::migrate_2_3()
 
 void BlockchainLMDB::migrate(const uint32_t oldversion)
 {
-  switch(oldversion) {
-  case 0:
-    migrate_0_1(); /* FALLTHRU */
-  case 1:
-    migrate_1_2(); /* FALLTHRU */
-  case 2:
-    migrate_2_3(); /* FALLTHRU */
-  default:
-    ;
-  }
+  if (oldversion < 1)
+    migrate_0_1();
+  if (oldversion < 2)
+    migrate_1_2();
+  if (oldversion < 3)
+    migrate_2_3();
 }
 
 }  // namespace cryptonote