Don't declare elf_sym_fns_lazy_psyms

An earlier patch neglected to delete a forward declaration of
elf_sym_fns_lazy_psyms.  This is no longer defined.  This patch
removes it.

gdb/ChangeLog
2021-03-27  Tom Tromey  <tom@tromey.com>

	* elfread.c (elf_sym_fns_lazy_psyms): Don't declare.

gdb/elfread.c

@@ -71,8 +71,6 @@ struct lazy_dwarf_reader : public psymbol_functions
   }
 };
 
-extern const struct sym_fns elf_sym_fns_lazy_psyms;
-
 /* The struct elfinfo is available only during ELF symbol table and
    psymtab reading.  It is destroyed at the completion of psymtab-reading.
    It's local to elf_symfile_read.  */