Don't add useless kill ring entries

Don't add useless kill ring entries

while describing snippet tables.  This should also help users of
packages like pbcopy.el (which creates a new process on every
modification of the kill-ring, hence many `kill-region' calls at once
cause Emacs to run out of file descriptors).

* yasnippet.el (yas--create-snippet-xrefs): Use delete-region instead of kill-region.

Copyright-paperwork-exempt: yes

Close #675.