kconfig: remove orphan lookup_file() declaration

There is no definition, no caller for lookup_file().
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>

scripts/kconfig/expr.h

@@ -279,7 +279,6 @@ struct jump_key {
 
 extern struct file *file_list;
 extern struct file *current_file;
-struct file *lookup_file(const char *name);
 
 extern struct symbol symbol_yes, symbol_no, symbol_mod;
 extern struct symbol *modules_sym;