Update RFC-001 to remove extraneous step

[jasonrudolph]

RFC-001 currently includes the following paragraph:

When a host closes a buffer, it will be removed from all guest portals. If another participant is working in that same buffer (i.e., not following the host), Teletype will first ask the host to confirm their intention to remove the buffer from the portal.

This pull request proposes that we remove the second sentence from that paragraph.

Our motivation for this change is as follows:

• If a host attempts to close a buffer that the guest is editing, Atom will detect that the buffer has unsaved changes, and Atom will automatically prompt the host to confirm their intention to close the buffer. If Teletype also attempts to confirm the host's intention to close the buffer, we'll essentially be prompting the host twice before they can close the buffer. 😬
• @as-cii and I have been using the current behavior (implemented in Allow guests to collaborate on multiple remote buffers (Part 1) #262) for a few weeks, and we haven't yet encountered a situation where this confirmation would have been useful.
• We may eventually want to allow guests to keep remote buffers open after the host closes them. If so, this confirmation would no longer be relevant and we'd need to remove it.

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Refs: #268

[nathansobo]

It's great to keep the docs up to date with our current thinking like this.