I agree with the two comments above. That canoe has a lot of cross-structural support for a 15' canoe: two seats and three thwarts.
You should be able to remove the quarter thwart behind the bow seat without any loss of structure because the bow seat itself is providing cross-structure at almost that very spot.
You could remove the center thwart and probably not affect structure noticeably. If you replace it with a gunwale-mounted center seat, that seat will replace the center thwart structure.
A different solution would be to sit on a bottom-mounted seat that you could build or buy. You could use this seat snugged up close to the center thwart without removing that thwart if you are a kneeler, or by removing the center thwart to give yourself more leg room for sitting. A three-thwarted 15' aluminum hull should be strong enough not to need a center thwart for recreational canoeing.