Remove deprecated alias io_service.

Author: chriskohlhoff

doc/requirements/AsyncRandomAccessReadDevice.qbk

@@ -27,7 +27,7 @@ boost_asio.reference.ReadHandler read handler] requirements.
     [
       Initiates an asynchronous operation to read one or more bytes of data
       from the device `a` at the offset `o`. The operation is performed via the
-      `io_service` object `a.get_io_service()` and behaves according to [link
+      I/O executor `a.get_executor()` and behaves according to [link
       boost_asio.reference.asynchronous_operations asynchronous operation]
       requirements.[br]
       [br]

doc/requirements/AsyncRandomAccessWriteDevice.qbk

@@ -27,7 +27,7 @@ boost_asio.reference.WriteHandler write handler] requirements.
     [
       Initiates an asynchronous operation to write one or more bytes of data to
       the device `a` at offset `o`. The operation is performed via the
-      `io_service` object `a.get_io_service()` and behaves according to [link
+      I/O executor `a.get_executor()` and behaves according to [link
       boost_asio.reference.asynchronous_operations asynchronous operation]
       requirements.[br]
       [br]

include/boost/asio.hpp

@@ -109,8 +109,6 @@
 #include <boost/asio/immediate.hpp>
 #include <boost/asio/io_context.hpp>
 #include <boost/asio/io_context_strand.hpp>
-#include <boost/asio/io_service.hpp>
-#include <boost/asio/io_service_strand.hpp>
 #include <boost/asio/ip/address.hpp>
 #include <boost/asio/ip/address_v4.hpp>
 #include <boost/asio/ip/address_v4_iterator.hpp>