Reduce max_concurrent_scans and max_concurrent_scans, remove unused instances

Hearing Aid profile is using one client, we can start one client less.

Bug: 79114848
(cherry picked from commit b52628d4362adcd05ce452716d15003a882307a8)
diff --git a/acts/tests/google/ble/scan/BleOpportunisticScanTest.py b/acts/tests/google/ble/scan/BleOpportunisticScanTest.py
index d407723..e907cbb 100644
--- a/acts/tests/google/ble/scan/BleOpportunisticScanTest.py
+++ b/acts/tests/google/ble/scan/BleOpportunisticScanTest.py
@@ -38,7 +38,7 @@
 
 class BleOpportunisticScanTest(BluetoothBaseTest):
     default_timeout = 10
-    max_scan_instances = 28
+    max_scan_instances = 27
     report_delay = 2000
     scan_callbacks = []
     adv_callbacks = []