coccinelle: fix verbose message about .cocci file being run

If you run coccicheck with V=1 and COCCI=, you will see a strange
path to the semantic patch file.  For example, run the following:

$ make V=1 COCCI=scripts/coccinelle/free/kfree.cocci coccicheck
  [ snip ]
 The semantic patch that makes this report is available
 in scriptcoccinelle/free/kfree.cocci.

Notice "s/" was dropped from "scripts/coccinelle/free/kfree.cocci".

When running coccicheck without O=, $srctree is expanded to ".", which
represents one arbitrary character in the regular expression.  Using
sed is not a good choice here.  Strip $srctree/ simply without sed.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Acked-by: Nicolas Palix <nicolas.palix@imag.fr>