[oni-org] Remove references to ‘oni-backlinks’

I haven't actually been using them at all, and they're just clogging up the
‘org-update-all-dblocks’ command by doing a bunch of unnecessary work.

oni-org/capture-templates/inbox-with-selection.org

@@ -7,6 +7,3 @@
 #+begin_quote
 %i
 #+end_quote
-
-#+BEGIN: oni-backlinks
-#+END

oni-org/capture-templates/reading-url-protocol.org

@@ -4,6 +4,3 @@
 :END:
 
 [[%:link]]
-
-#+BEGIN: oni-backlinks
-#+END

oni-org/capture-templates/reading-url.org

@@ -2,6 +2,3 @@
 :PROPERTIES:
 :CREATED:  %U
 :END:
-
-#+BEGIN: oni-backlinks
-#+END

oni-org/capture-templates/task.org

@@ -2,6 +2,3 @@
 :PROPERTIES:
 :CREATED:  %U
 :END:
-
-#+BEGIN: oni-backlinks
-#+END