doc: remove FIXME simplify uploading

there's no need to simplify the uploading; the uploading is not on
within this one future which will be executed so it's not safe to just
remove it or revert it back.

also I am hesitant to add new changes at this point, the diff is large
enough.

pageserver/src/tenant.rs

@@ -1440,7 +1440,6 @@ impl Tenant {
 
         let span = tracing::Span::current();
 
-        // FIXME: simplify uploading
         // if we have concurrent requests, we will only execute one version of following future;
         // initially it did not have any means to be cancelled.
         //