qt: Disconnect CClientUIInterface from SplashScreen::ConnectWallet
qt: Disconnect CClientUIInterface from SplashScreen::ConnectWallet

Due to not disconnecting the signal there the trace of the `loadwallet` 
rpc command ends 
in a call of `SplashScreen::ConnectWallet` for the not longer existent 
splashscreen object.

https://github.com/dashpay/dash/blob/7770a7da959dfd4c06c6e4b32fb70c0c7bb59085/src/wallet/wallet.cpp#L4192

Wasn't an issue before the `loadwallet` rpc was introduced because 
`CWallet::LoadWallet` was only called once on startup.

dashpay/dash,UdjinM6/dash,dashpay/dash,UdjinM6/dash,thelazier/dash,UdjinM6/dash,dashpay/dash,UdjinM6/dash,thelazier/dash,UdjinM6/dash,thelazier/dash,thelazier/dash,dashpay/dash,dashpay/dash,thelazier/dash