2 switch to one - Similar but not the same
The black wire going into the groups of black wires with the red wire nut is your feed. The solo black and red wires were your travelers to the other switch. Connect your feed to one of the travelers. Try your other switch. If the switch position is correct, toggle up is on, toggle down is off, you're good to go, cap the unused traveler. If not, then connect the feed to the other traveler and cap the unused one.
Put a blank box cover over the box without the switch. It has to remain accessible and can't be drywalled over.