bfd: Discard symbol regardless of warning flag

The discard of symbols should be performed whether the warning for
the discard is enabled or not.
Without this patch, ld would segfault in bfd_section_removed_from_list,
called in the if-statement right after this block, as the argument
isec->output_section can be NULL.

Signed-off-by: Torbjörn SVENSSON <torbjorn.svensson@foss.st.com>
Co-Authored-By: Yvan ROUX <yvan.roux@foss.st.com>