coccinelle: Remove unnecessary parentheses for only one possible change.

The parentheses are only needed if there is a disjunction, ie a
set of possible changes. If there is only one pattern, we can
remove these parentheses. Just like the format:

  -  x
  +  y

not:

  (
  -  x
  +  y
  )

Signed-off-by: Hongbo Li <lihongbo22@huawei.com>
Signed-off-by: Julia Lawall <Julia.Lawall@inria.fr>