Remove is:dm search suggestion for spectators

[alya]

Typing is: in the search bar shows the following suggestion for spectators:

Since spectators do not have DMs, we should remove it, as we do for is:starred, etc.

[zulipbot]

Hello @zulip/server-search members, this issue was labeled with the "area: search" label, so you may want to check it out!

[joyhchen]

@zulipbot claim

[zulipbot]

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