Remove reference to future Committee Note

Section 5.4.11.4 makes reference to (and could be read as a promise to the delivery of) a future supplemental publication.

"An MQTT supplemental publication: MQTT security standards will provide further information related to the usage of various industry security frameworks and standards."

Section 5.2 already provides (as agreed by the TC) a hard reference and link to the Security SC Committee Note on the NIST Cyber Security Framework. The aim is to publish this document alongside the core spec, so the link is valid.

The TC will likely wish to publish a number of Committee Notes and supplemental documents as we move forward (most likely post standardization). I believe the convention is for such publications to point to the core spec and for the list of references in the core spec to be periodically refreshed. As such, the sentence seems redundant.