coccinelle: device_attr_show: Remove useless expression STR

Commit ff82e84e80fc ("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 <[e-mail]>
CC: Nicolas Palix <[e-mail]>
CC: [e-mail]
Fixes: ff82e84e80fc ("coccinelle: device_attr_show: simplify patch case")
Signed-off-by: Li Zhijian <[e-mail]>
Signed-off-by: Julia Lawall <[e-mail]>

scripts/coccinelle/api/device_attr_show.cocci

···
 
 @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)