le_audio: remove gatt_read_multi_op_simulate

Using gatt_read_multi_op_simulate, that was waiting for all Read
operations to complete before returning read values, lead to a bug when
read value was updated by notification before function returned.
If EATT is not supported by peer, reads should pass read values as soon
as read response is received. This patch fixes following scenario:
1. Read Req sent for handle A
2. Read Rsp received for handle A
3. Read Req sent for handle B
4. Notification received for handle A
5. Read Rsp received for handle B
Value from step 2 would be overwritten by value from step 4

Bug: 369970309
Flag: EXEMPT, day-to-day bugfix
Test: atest --host --no-bazel-mode bluetooth_le_audio_client_test
Change-Id: Ie90e94ef8777a16dd2792fb06c13e96bae5e68c1