Remove flakey designation for testStartUser_background

Test is marked flakey, but seems to be stable now. Remove the annotation unless
we find out that it is still flakey.

Fixes: 118932054
Test: atest com.android.server.am.UserControllerTest#testStartUser_background
Change-Id: Ib768aa861fd1de6312519e4f7b24af258550078a