Remove --param max-fsm-thread-length

This removes max-fsm-thread-length which is obsoleted by
max-jump-thread-paths.

	* doc/invoke.texi (max-fsm-thread-length): Remove.
	* params.opt (max-fsm-thread-length): Likewise.
	* tree-ssa-threadbackward.cc
	(back_threader_profitability::profitable_path_p): Do not
	check max-fsm-thread-length.