Since this "extension" does not exist (on the other hand, 'Zve64d' exists)
and it's not useful if we keep it (as other code portions just ignore
"zve32d"), this commit just removes it.
bfd/ChangeLog:
* elfxx-riscv.c (riscv_supported_std_z_ext): Remove 'Zve32d'
extension from the list.