cloud_storage: Clean up segment hydration

When segment is hydrated, we are guaranteed to be in legacy mode, which
means that we were not able to download an index from cloud storage.
The change cleans up the code path in segment hydration, so that if a
segment is downloaded, an index is always created on the fly from the
segment data.

src/v/cloud_storage/remote_segment.cc

@@ -495,13 +495,11 @@ ss::future<> remote_segment::do_hydrate_segment() {
       _bucket,
       _path,
       [this, &reservation](uint64_t size_bytes, ss::input_stream<char> s) {
-          if (is_legacy_mode_engaged()) {
-              return put_segment_in_cache_and_create_index(
-                size_bytes, reservation, std::move(s));
-          } else {
-              return put_segment_in_cache(
-                size_bytes, reservation, std::move(s));
-          }
+          // Always create the index because we are in legacy mode if we ended
+          // up hydrating the segment. Legacy mode indicates a missing index, so
+          // we create it here on the fly using the downloaded segment.
+          return put_segment_in_cache_and_create_index(
+            size_bytes, reservation, std::move(s));
       },
       local_rtc);