A few comment cleanups

A few comment cleanups

I stumbled upon a few comments that I think are outdated.

Comment for elfread.c (elf_symfile_init): As far as history goes in git,
I don't see anything related to that.

Comment for elfread.c (elf_symfile_read): References a parameter that was
removed in 1999.

Comment for struct sym_fns/sym_offsets: References a parameter that was
changed in 1999.

gdb/ChangeLog:

	* elfread.c (elf_symfile_init): Remove stale comment.
	(elf_symfile_read): Same.
	* symfile.h (struct sym_fns): Same.