Avoid unnecessary multiple output buffers in admin-ajax get-comments

[mitchoyoshitaka]

In the admin-ajax get-comments code, there's currently a loop over each list table item in order to build up the comments HTML, but it creates, gets, and clears the output buffer for each of these loops. This probably could be collapsed into one output buffer?

[nacin]

In [17897]:

Move output buffering out of a loop, collapsing it into one buffer. props mitchoyoshitaka, fixes #17352.