Remove redundant check for slot->allocated

This check became redundant when support for direct access to key
slots was removed.
diff --git a/library/psa_crypto.c b/library/psa_crypto.c
index d272334..554da4b 100644
--- a/library/psa_crypto.c
+++ b/library/psa_crypto.c
@@ -899,8 +899,6 @@
     status = psa_get_key_slot( handle, &slot );
     if( status != PSA_SUCCESS )
         return( status );
-    if( ! slot->allocated )
-        return( PSA_ERROR_INVALID_HANDLE );
 
     return( psa_wipe_key_slot( slot ) );
 }