org-capture-kill uses the kill ring

[Allen Li <vianchielfaura@gmail.com>]

org-capture-kill (C-c C-k) used to abort capturing puts the contents
of the capture buffer into the kill ring.

This is obstructive when the user wants to abort a capture, but save a
portion of the text into the kill ring to yank elsewhere.  The aborted
capture contents will replace the desired content.

Aborting capture should act transparently like killing a buffer.  It
shouldn't affect the kill ring.  If the user wants to keep the capture
contents, he can trivially run C-x h mark-whole-buffer M-w
kill-ring-save