Here's the rub - there appears to be no lintel above the window, but there are bricks and there is a roof which means that rotten or not that window and quite probably the sill beneath it, are load bearing. So, if you cut away the sill, depending on how the window has been fixed in place and how many fixings there are, there may well be no support for the window. You just can't tell. In view of the fact that the window has been ""cowboyed"-in, I wouldn't want to put any faith in how well it has been fixed. If it isn't adequately fixed you could find yourself, having removed the sill, with broken glass, cracked brickwork and possibly a window frame/glass/bricks/concrete lintel/roofing* in your lap (or on your head) should the window frame move. On the other hand you might get away with it, so in the words of Dirty Harry, "... ask yourself, punk, are you feeling lucky?"
Personally, I wouldn't risk it - and that's advice from someone who regularly props roofs, beams, floors and joists in old buildings much bigger and heavier than this. You either need to do the job correctly, or not at all. The idea is that you go home safely every night and in one piece.
* = delete as appropriate