Disable readstream's reliance on seqscan readahead (#9860)

Neon doesn't have seqscan detection of its own, so stop read_stream from
trying to utilize that readahead, and instead make it issue readahead of
its own.

## Problem

@knizhnik noticed that we didn't issue smgrprefetch[v] calls for
seqscans in PG17 due to the move to the read_stream API, which assumes
that the underlying IO facilities do seqscan detection for readahead.
That is a wrong assumption when Neon is involved, so let's remove the
code that applies that assumption.

## Summary of changes
Remove the cases where seqscans are detected and prefetch is disabled as
a consequence, and instead don't do that detection.

PG PR: https://github.com/neondatabase/postgres/pull/532

vendor/revisions.json

@@ -1,7 +1,7 @@
 {
   "v17": [
     "17.2",
-    "471c449ab8f8ff5988b6bfb9eafa0a79772ad562"
+    "01fa3c48664ca030cfb69bb4a350aa9df4691d88"
   ],
   "v16": [
     "16.6",