Simplify async pool handling

A lot of the pool handling code was in the arch specific files, but was
actually boiler plate and the same across the implementations. This commit
moves as much code as possible out of the arch specific files.

Reviewed-by: Rich Salz <rsalz@openssl.org>

crypto/async/arch/async_null.c

@@ -56,41 +56,6 @@
 
 #ifdef ASYNC_NULL
 
-STACK_OF(ASYNC_JOB) *async_get_pool(void)
-{
-    return NULL;
-}
-
-int async_set_pool(STACK_OF(ASYNC_JOB) *poolin, size_t curr_size,
-                   size_t max_size)
-{
-    return 0;
-}
-
-void async_increment_pool_size(void)
-{
-    return;
-}
-
-void async_release_job_to_pool(ASYNC_JOB *job)
-{
-    return;
-}
-
-size_t async_pool_max_size(void)
-{
-    return 0;
-}
-
-void async_release_pool(void)
-{
-    return;
-}
-
-int async_pool_can_grow(void) {
-    return 0;
-}
-
 int async_pipe(OSSL_ASYNC_FD *pipefds)
 {
     return -1;