To uninstall Defender follow this -
1) Open the bitdefender mobile security app
.
2) Navigate to Menu > Account info.
3) Tap on Uninstall and enter your PIN.
4) Confirm the uninstall and it will get done.
However I don't suggest you to uninstall the antivirus rather only remove the SpinnerBoost app you've mentioned. It is a known virus and should be removed immediately. Keep the antivirus.