cred: remove unused get_new_cred()

This helper is not used anymore so remove it.

Link: https://lore.kernel.org/r/[e-mail]
Reviewed-by: Jeff Layton <[e-mail]>
Reviewed-by: Jens Axboe <[e-mail]>
Signed-off-by: Christian Brauner <[e-mail]>

Documentation/security/credentials.rst

···
      This gets a reference on a live set of credentials, returning a pointer to
      that set of credentials.
 
- - ``struct cred *get_new_cred(struct cred *cred);``
-
-     This gets a reference on a set of credentials that is under construction
-     and is thus still mutable, returning a pointer to that set of credentials.
-
 
 Open File Credentials
 =====================

include/linux/cred.h

···
 }
 
 /**
- * get_new_cred - Get a reference on a new set of credentials
- * @cred: The new credentials to reference
- *
- * Get a reference on the specified set of new credentials.  The caller must
- * release the reference.
- */
-static inline struct cred *get_new_cred(const struct cred *cred)
-{
-	struct cred *nonconst_cred = (struct cred *) cred;
-	return get_new_cred_many(nonconst_cred, 1);
-}
-
-/**
  * get_cred_many - Get references on a set of credentials
  * @cred: The credentials to reference
  * @nr: Number of references to acquire