The MBR contains 2 things of interest here: a data structure describing the partition layout, and the 1st-stage booter code. When you rig a system with GRUB that 1st-stage MBR booter code gets rewritten overwritten with GRUB's 1st-stage. On a "dual boot" system that 1st-stage GRUB booter code is rigged to blindly assume it can find it its subsequent stages elsewhere in the disk, typically in one of the Linux partitions. By deleting the Linux partitions you pulled the rug out from under GRUB's 1st-stage which is still present in the MBR (as is the intact partition layout info) so, as mentioned already, replacing that GRUB code in the MBR with the Windows booter is what's wanted, and that's accomplished with a Windows "Fix MBR" maneuver, an exercise left to the reader...

The MBR contains 2 things of interest here: a data structure describing the partition layout, and the 1st-stage booter code. When you rig a system with GRUB that 1st-stage MBR booter code gets rewritten with GRUB's 1st-stage. On a "dual boot" system that 1st-stage GRUB booter code is rigged to blindly assume it can find it subsequent stages elsewhere in the disk, typically in one of the Linux partitions. By deleting the Linux partitions you pulled the rug out from under GRUB's 1st-stage which is still present in the MBR (as is the partition layout info) so, as mentioned already, replacing that GRUB code in the MBR with the Windows booter is what's wanted, and that's accomplished with a Windows "Fix MBR" maneuver, an exercise left to the reader...