Clear flag - emergency_callback_mode_notification

This commit removes the feature flag emergency_callback_mode_notification and its associated code.
The flag has been fully launched and is now in the 'Eligible For Cleanup' stage.
This change cleans up the flag checks and related dead code to simplify the codebase.

Bug: 361014075
Bug: 359064059
Test: atest CtsTelephonyTestCases:TelephonyCallbackTest,
      atest CtsTelephonyTestCases:TelephonyRegistryManagerTest
Test: atest TelephonyRegistryTest, atest DefaultPhoneNotifierTest,
      atest EmergencyStateTrackerTest
Flag: EXEMPT removing <com.android.internal.telephony.flags.emergency_callback_mode_notification>
Change-Id: I6825b4a7fd8c543cc07c5054fa9506af5a0e373b