Rm defer_show_soft_input_until_session_creation

This deletes the flag and flagged off code as it has been enabled for
a full release already.

Flag: EXEMPT flag removal
Bug: 409605050
Test: atest InputMethodStartInputLifecycleTest#testShowingImeDuringSessionChange_doesntShowOnPreviousSession
Change-Id: I6d4718aac603778c7e60c88c3eab941972693b69