Remove obsolete mentions of PSA_ERROR_EMPTY_SLOT

There are no more "empty slots", so finish removing the corresponding
error.
diff --git a/include/psa/crypto.h b/include/psa/crypto.h
index f099967..3036d17 100644
--- a/include/psa/crypto.h
+++ b/include/psa/crypto.h
@@ -1210,7 +1210,6 @@
  * \retval #PSA_SUCCESS
  *         Success.
  * \retval #PSA_ERROR_INVALID_HANDLE
- * \retval #PSA_ERROR_EMPTY_SLOT
  * \retval #PSA_ERROR_NOT_PERMITTED
  * \retval #PSA_ERROR_INVALID_ARGUMENT
  *         \p handle is not compatible with \p alg.
@@ -1249,7 +1248,6 @@
  *         The MAC of the message was calculated successfully, but it
  *         differs from the expected value.
  * \retval #PSA_ERROR_INVALID_HANDLE
- * \retval #PSA_ERROR_EMPTY_SLOT
  * \retval #PSA_ERROR_NOT_PERMITTED
  * \retval #PSA_ERROR_INVALID_ARGUMENT
  *         \p handle is not compatible with \p alg.
@@ -1596,7 +1594,6 @@
  * \retval #PSA_SUCCESS
  *         Success.
  * \retval #PSA_ERROR_INVALID_HANDLE
- * \retval #PSA_ERROR_EMPTY_SLOT
  * \retval #PSA_ERROR_NOT_PERMITTED
  * \retval #PSA_ERROR_INVALID_ARGUMENT
  *         \p handle is not compatible with \p alg.
@@ -1638,7 +1635,6 @@
  * \retval #PSA_SUCCESS
  *         Success.
  * \retval #PSA_ERROR_INVALID_HANDLE
- * \retval #PSA_ERROR_EMPTY_SLOT
  * \retval #PSA_ERROR_NOT_PERMITTED
  * \retval #PSA_ERROR_INVALID_ARGUMENT
  *         \p handle is not compatible with \p alg.
@@ -2214,7 +2210,6 @@
  * \retval #PSA_SUCCESS
  *         Success.
  * \retval #PSA_ERROR_INVALID_HANDLE
- * \retval #PSA_ERROR_EMPTY_SLOT
  * \retval #PSA_ERROR_NOT_PERMITTED
  * \retval #PSA_ERROR_INVALID_ARGUMENT
  *         \p handle is not compatible with \p alg.
@@ -2276,7 +2271,6 @@
  * \retval #PSA_SUCCESS
  *         Success.
  * \retval #PSA_ERROR_INVALID_HANDLE
- * \retval #PSA_ERROR_EMPTY_SLOT
  * \retval #PSA_ERROR_NOT_PERMITTED
  * \retval #PSA_ERROR_INVALID_ARGUMENT
  *         \p handle is not compatible with \p alg.
@@ -3384,7 +3378,6 @@
  * \retval #PSA_SUCCESS
  *         Success.
  * \retval #PSA_ERROR_INVALID_HANDLE
- * \retval #PSA_ERROR_EMPTY_SLOT
  * \retval #PSA_ERROR_NOT_PERMITTED
  * \retval #PSA_ERROR_INVALID_ARGUMENT
  *         \p alg is not a key agreement algorithm