crypto: asymmetric_keys - Remove unused key_being_used_for[]

key_being_used_for[] is an unused array of textual names for
the elements of the enum key_being_used_for.  It was added in 2015 by
commit 99db4435 ("PKCS#7: Appropriately restrict authenticated
attributes and content type")

Remove it.

Signed-off-by: Dr. David Alan Gilbert <linux@treblig.org>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>