RPM will prompt to install our signing key, so no need to manually import it:
```
ZFS on Linux for EL9 - kmod 3.3 MB/s | 3.4 kB 00:00
Importing GPG key 0x9DB84141:
Userid : "OpenZFS <release@openzfs.org>"
Fingerprint: 7DC7 299D CF7C 7FD9 CD87 701B A599 FD5E 9DB8 4141
From : /etc/pki/rpm-gpg/RPM-GPG-KEY-openzfs
Is this ok [y/N]: y
```
Removing the line also gets around the problem of the key being renamed from `RPM-GPG-KEY-zfsonlinux` -> `RPM-GPG-KEY-openzfs`
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