pg_dump: Remove unnecessary code for security labels on extensions.

pg_dump: Remove unnecessary code for security labels on extensions.

Commit d9572c4e3b4 added extension support and made pg_dump attempt to
dump security labels on extensions. However, security labels on extensions
are not actually supported, so this code was unnecessary.

This commit removes it.

Suggested-by: Jian He <jian.universality@gmail.com>
Author: Fujii Masao <masao.fujii@gmail.com>
Reviewed-by: Jian He <jian.universality@gmail.com>
Discussion: https://postgr.es/m/CACJufxF8=z0v=888NKKEoTHQ+Jc4EXutFi91BF0fFjgFsZT6JQ@mail.gmail.com