scripts: cstyle: remove unused -C

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Ahelenia Ziemiańska <nabijaczleweli@nabijaczleweli.xyz>
Closes #13285

man/man1/cstyle.1

@@ -73,10 +73,6 @@ Used as part of the putback checks.
 Verbose output; includes the text of the line of error, and, for
 .Fl c ,
 the first statement in the current continuation block.
-.It Fl C
-Ignore errors in header comments (i.e. block comments starting in the
-first column).
-Not generally used.
 .It Fl P
 Check for use of non-POSIX types.
 Historically, types like