RISC-V: Remove dead perm series code and document.

RISC-V: Remove dead perm series code and document.

With the introduction of shuffle_series_patterns the explicit handler
code for a perm series is dead.  This patch removes it and also adds
a function-level comment to shuffle_series_patterns.

gcc/ChangeLog:

	* config/riscv/riscv-v.cc (expand_const_vector): Document.
	(shuffle_extract_and_slide1up_patterns): Remove.