Fix some comments related to library unloading

Michael Paquier <michael@paquier.xyz>

Commit: 7f97b4734f937db6f8dab1bbf8bbaab349e6c9b1
Author: Michael Paquier <michael@paquier.xyz>
Date: 2024-12-23T05:46:49Z
Releases: 18.0
Fix some comments related to library unloading

Library unloading has never been supported with its code removed in
ab02d702ef08, and there were some comments still mentioning that it was
a possible operation.

ChangAo has noticed the incorrect references in dfmgr.c, while I have
noticed the other ones while scanning the rest of the tree for similar
mistakes.

Author: ChangAo Chen, Michael Paquier
Reviewed-by: Tom Lane
Discussion: https://postgr.es/m/tencent_1D09840A1632D406A610C8C4E2491D74DB0A@qq.com

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