How to convert encrypted wallet.dat to decrypted? (Yes, I know the password) I don't want to spend transaction fee for transferring coins to unencrypted wallet.dat.

What should I do?
The easiest way is to export your private keys using the dumpprivkey functionality. 


  DumpPrivKey: returns the wallet-import-format (WIP) private key corresponding to an address. (But does not remove it from the wallet.) Bitcoin Developer Reference


Do this for each address that is storing bitcoin (use listreceivedbyaddress to view all addresses with funds) 


  ListReceivedByAddress: lists the total number of bitcoins received by each address. Bitcoin Developer Reference


Then create a backup copy of your wallet.dat file and then remove it.  The next time bitcoin-qt runs it will create a new empty unencrypted wallet.  Now you can use importprivkey to restore your funds. 


  ImportPrivKey: adds a private key to your wallet. The key should be formatted in the wallet import format created by the dumpprivkey RPC. Bitcoin Developer Reference