Open regedit and go to:
HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\Video\
In there, there'll be some keys looking like this:
{XXXXXXXX-XXXX-XXXX-XXXX-XXXXXXXXXXXX}Look for keys that begin with DALR6 DFPXXXXxYYYYx0xFF
XXXX= Horizontal resolution
YYYY= Vertical resolution
FF= Display frequency
Click on the key matching the screen resolution and frequency you want to remove the borders of.
A lot of the key will consist of "00", but there'll be a few non-00 entries. Change all of these to 00.
There may be two subfolders, 0000 and 0001, containing mirrors of the same keys. When you change a key in one subfolder, it will also change in the other.