You deleted Ubuntu partition => you deleted second stage of GRUB.
First stage of GRUB is still in control of the booting as it occupies the MBR (Master Boot Record) I suppose (let's ignore UEFI for this solution to simplify things).
To fix Windows 8 boot you need to execute two commands when booted from Windows 8 installation/recovery USB: (on command prompt)
bootsect /nt60 ALL /mbr
bootsect writes boot code only to "usual" Windows partitions (e.g. NTFS, FAT) and MBR, does not write boot code to Linux partitions (not relevant in this case).
bcdboot N:\windows /s c:
assuming c: is active partition, N: is where Windows is installed.
You can use diskpart.exe to get info about active partition on hard disk with:
list disk select disk #num - (#num = number of hard disk) list vol
you get a list of volumes - volume (with or without drive letter) with "System" in last column is active.
You can map a volume like this:
sel vol #num - (#num = number of active) assign LETTER=R
assigns drive letter R: to active.