nptl_db: Clean up main/rtld variable handling

Most symbols are now in libc.so.6.  The "main" (exempted from
coverage checks) status is therefore not necessary.  Use
DB_MAIN_VARIABLE for the remaining separate symbol,
__nptl_initial_report_events.  DB_RTLD_VARIABLE is now unused, so
remove it.

Reviewed-by: Carlos O'Donell <carlos@redhat.com>
Tested-by: Carlos O'Donell <carlos@redhat.com>