To restore Win 10 default bootloader follow these steps:
c:\> bootsect /nt60 <drive name>: /mbr
<drive name> is the drive letter where the Master Boot Record (MBR) will be updated
For example to update C master boot record this is the command:
c:\> bootsect /nt60 c: /mbr
For more help about bootsect command see here - https://docs.microsoft.com/en-us/windows-hardware/manufacture/desktop/bootsect-command-line-options