Reduce Coccinelle choices in string_choices.cocci

The isomorphism neg_if_exp negates the test of a ?: conditional,
making it unnecessary to have an explicit case for a negated test
with the branches inverted.

At the same time, we can disable neg_if_exp in cases where a
different API function may be more suitable for a negated test.

Finally, in the non-patch cases, E matches an expression with
parentheses around it, so there is no need to mention ()
explicitly in the pattern.  The () are still needed in the patch
cases, because we want to drop them, if they are present.

Signed-off-by: Julia Lawall <Julia.Lawall@inria.fr>