coccinelle: device_attr_show: Remove useless expression STR

Commit ff82e84e ("coccinelle: device_attr_show: simplify patch case")
simplifies the patch case, as a result, STR is no longer needed.

This also helps to fix below coccicheck warning:
> warning: rp: metavariable STR not used in the - or context code

CC: Julia Lawall <Julia.Lawall@inria.fr>
CC: Nicolas Palix <nicolas.palix@imag.fr>
CC: cocci@inria.fr
Fixes: ff82e84e ("coccinelle: device_attr_show: simplify patch case")
Signed-off-by: Li Zhijian <lizhijian@fujitsu.com>
Signed-off-by: Julia Lawall <Julia.Lawall@inria.fr>

scripts/coccinelle/api/device_attr_show.cocci

@@ -28,7 +28,7 @@ ssize_t show(struct device *dev, struct device_attribute *attr, char *buf)
 
 @rp depends on patch@
 identifier show, dev, attr, buf;
-expression BUF, SZ, FORMAT, STR;
+expression BUF, SZ, FORMAT;
 @@
 
 ssize_t show(struct device *dev, struct device_attribute *attr, char *buf)