Don't clear 'qf' in elf_symfile_read

Don't clear 'qf' in elf_symfile_read

I noticed that I forgot to make a change in my series to make it
possible to attach multiple debug readers to an objfile.  In one spot,
elf_symfile_read still clears the 'qf' list.  However, this should
have been removed toward the end of that series.

This patch fixes the offending spot.  Tested on x86-64 Fedora 32.

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

	* elfread.c (elf_symfile_read): Don't clear 'qf'.