I can't test it since I don't have a partitioned router available.
"Usage Guidelines For the Cisco 1600 series and Cisco 3600 series routers, to undo partitioning, use the partition flash-filesystem:1 or no partition flash-filesystem: command. For other Class B platforms, use either the partition flash 1 or no partition flash command. If there are files in a partition other than the first, you must use the erase flash-filesystem:partition-number command to erase the partition before reverting to a single partition."
I tried that command much earlier. It only works (on an 800 series router - with class B flash) if you have flash on the board, PLUS a Strata flash module in the extra slot, or, have previously partitioned your flash on the board:
837#config t Enter configuration commands, one per line. End with CNTL/Z.
837(config)#no partition flash
837# sho flash