btrfs: Drop stray check of fixup_workers creation

The issue was introduced in a79b7d4b3e8118f265dcb4bdf9a572c392f02708,
adding allocation of extent_workers, so this stray check is surely not
meant to be a check of something else.

Bugzilla: https://bugzilla.kernel.org/show_bug.cgi?id=82021
Reported-by: Maks Naumov <maksqwe1@ukr.net>
Signed-off-by: Andrey Utkin <andrey.krieger.utkin@gmail.com>
Reviewed-by: Eric Sandeen <sandeen@redhat.com>
Signed-off-by: Chris Mason <clm@fb.com>