bash doesn't create named pipe filesystem objects when you use process substitution, it passes file descriptors via the dynamically created kernel managed /dev/fd paths. There is nothing to clean up.
Oh, I think I see what you're saying. So calling them named pipes is inaccurate, then. If I'm not mistaken, using features of the /dev filesystem would be dependent on Linux, then. Either way, it isn't really a very robust design. Not like a pipe returned by pipe(2) and passed to a program that is expecting it. Simple unnamed pipes are a fundamental part of unix.
Here's what the man page says:
Process substitution is supported on systems that support named pipes (FIFOs) or the /dev/fd method of
naming open files. It takes the form of <(list) or >(list). The process list is run with its input or
output connected to a FIFO or some file in /dev/fd. The name of this file is passed as an argument to
the current command as the result of the expansion. If the >(list) form is used, writing to the file
will provide input for list. If the <(list) form is used, the file passed as an argument should be read
to obtain the output of list.
So it's done with /dev/fd if that's available, which basically means Linux with that filesystem mounted. Otherwise it falls back on named pipes.
