coff-sh.c keep_relocs, keep_contents and keep_syms

keep_relocs and keep_contents are unused nowadays except by
xcofflink.c, and I can't see a reason why keep_syms needs to be set.
The external syms are read and used by sh_relax_section and used by
sh_relax_delete_bytes.  There doesn't appear to be any way that
freeing them will cause trouble.

	* coff-sh.c (sh_relax_section): Don't set keep_relocs,
	keep_contents or keep_syms.
	(sh_relax_delete_bytes): Don't set keep_contents.