Hello Thesosman, and welcome to Seven Forums.
It depends on if the OS you removed from the Windows Boot Manager screen is the one you installed first or not, and if it's the boot partition.
If not, then you could usually just delete the partition
that OS is on to be unallocated space and extend
the other partition into it.
If it is, then please post back a screenshot
of your Disk Management (diskmgmt.msc) showing all of your drives layout, and what OS was removed from the list.
Hope this helps,