cocci: remove 'unused.cocci'

When 'unused.cocci' was added in 4f40f6c (cocci: add and apply a
rule to find "unused" strbufs, 2022-07-05) it found three unused
strbufs, and when it was generalized in the next commit it managed to
find an unused string_list as well.  That's four unused variables in
over 17 years, so apparently we rarely make this mistake.

Unfortunately, applying 'unused.cocci' is quite expensive, e.g. it
increases the from-scratch runtime of 'make coccicheck' by over 5:30
minutes or over 160%:

  $ make -s cocciclean
  $ time make -s coccicheck
      * new spatch flags

  real    8m56.201s
  user    0m0.420s
  sys     0m0.406s
  $ rm contrib/coccinelle/unused.cocci contrib/coccinelle/tests/unused.*
  $ make -s cocciclean
  $ time make -s coccicheck
      * new spatch flags

  real    3m23.893s
  user    0m0.228s
  sys     0m0.247s

That's a lot of runtime spent for not much in return, and arguably an
unused struct instance sneaking in is not that big of a deal to
justify the significantly increased runtime.

Remove 'unused.cocci', because we are not getting our CPU cycles'
worth.

Signed-off-by: SZEDER Gábor <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: szeder