Remove unused crypt_token_external_support.

API can expose same information with crypt_token_external_path.

Also add print out of external token in cryptsetup --help action
(if enabled)

lib/libcryptsetup.h

@@ -2254,13 +2254,6 @@ typedef struct  {
  */
 int crypt_token_register(const crypt_token_handler *handler);
 
-/**
- * Report external token handlers (plugins) support
- * 
- * @return @e 0 when enabled or negative errno value otherwise.
- */
-int crypt_token_external_support(void);
-
 /**
  * Report configured path where library searches for external token handlers
  *