makefs: Remove impossible if condition

This test is inside the else block for `if (*oldname == '.')`, so
*oldname cannot be '.' here.

Reviewed by:	kevans
Sponsored by:	The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D48243

(cherry picked from commit 788e1eb7c1)

usr.sbin/makefs/cd9660.c

@@ -1683,12 +1683,8 @@ cd9660_level2_convert_filename(iso9660_disk *diskStructure, const char *oldname,
 			else if (isupper((unsigned char)*oldname) ||
 			    isdigit((unsigned char)*oldname))
 				*newname++ = *oldname;
-			else if (diskStructure->allow_multidot &&
-			    *oldname == '.') {
-				*newname++ = '.';
-			} else {
+			else
 				*newname++ = '_';
-			}
 
 			if (found_ext)
 				extlen++;