Removing systray icon will not affect WD's functionality. You can use Autoruns, it will show it for sure.
Code:
reg delete "HKLM\Software\Microsoft\Windows\CurrentVersion\Explorer\StartupApproved\Run" /v "Windows Defender" /f
reg delete "HKCU\Software\Microsoft\Windows\CurrentVersion\Run" /v "Windows Defender" /f
reg delete "HKLM\Software\Microsoft\Windows\CurrentVersion\Run" /v "WindowsDefender" /f