remove suggestion for pem section for keyrings

[1000TurquoisePogs]

I am removing a section that suggests users define zowe.certificate.pem when using multiple CAs and using a keyring.
The pem section has been and is still required for PKCS12, at all, but in the case of keyrings, it was needed only up until v2.9 (see https://docs.zowe.org/stable/whats-new/release-notes/v2_9_0#zlux-server-framework)

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