[chore] Remove max_in_flight_size_mib option (#36707)

#### Description
Removes `max_in_flight_size_mib` in the Readme since it has been
replaced by `admission` settings in the otelarrow receiver.

Opted not to add the `admission` options since the config was focused on
the exporter/processor but feel free to close if that's the better
route.

Co-authored-by: Alex Boten <223565+codeboten@users.noreply.github.com>
Co-authored-by: Andrzej Stencel <andrzej.stencel@elastic.co>

exporter/otelarrowexporter/README.md

@@ -305,5 +305,4 @@ processors:
     send_batch_max_size: 1500
     send_batch_size: 1000
     timeout: 1s
-    max_in_flight_size_mib: 128
 ```