Remove "allocated" flag from key slots

The flag to mark key slots as allocated was introduced to mark slots
that are claimed and in use, but do not have key material yet, at a
time when creating a key used several API functions: allocate a slot,
then progressively set its metadata, and finally create the key
material. Now that all of these steps are combined into a single
API function call, the notion of allocated-but-not-filled slot is no
longer relevant. So remove the corresponding flag.

A slot is occupied iff there is a key in it. (For a key in a secure
element, the key material is not present, but the slot contains the
key metadata.) This key must have a type which is nonzero, so use this
as an indicator that a slot is in use.
diff --git a/library/psa_crypto_slot_management.h b/library/psa_crypto_slot_management.h
index 049520d..cde590f 100644
--- a/library/psa_crypto_slot_management.h
+++ b/library/psa_crypto_slot_management.h
@@ -58,13 +58,13 @@
  * This does not affect persistent storage. */
 void psa_wipe_all_key_slots( void );
 
-/** Find a free key slot and mark it as in use.
+/** Find a free key slot.
+ *
+ * This function returns a key slot that is available for use and is in its
+ * ground state (all-bits-zero).
  *
  * \param[out] handle   On success, a slot number that can be used as a
- *                      handle to the slot. The selected slot was not
- *                      in use before. This function marks it as in use
- *                      and otherwise leaves it in a freshly-initialized
- *                      state.
+ *                      handle to the slot.
  * \param[out] p_slot   On success, a pointer to the slot.
  *
  * \retval #PSA_SUCCESS