Remove flag for boot completed fgs restrictions.

The feature is now completely rolled out.

Fixes: 409605469
Test: atest BootCompletedFgsStartTest
Test: atest ServiceTest
Flag: EXEMPT removing com.android.server.am.fgs_boot_compelted
Change-Id: I48b33b1fa51fb8e1358bd8ac1e1b22ddd5ffa0fe