Remove magic code to flash the screen

The code is originally introduced in aosp/87740 , not sure why
it's necessary to do `blank(true); blank(false);` . Partners have
complained that such code causes device's screen to flicker, so remove
it.

Test: th
Bug: 194757527
Change-Id: I271883ea8c96a5612283aa34806b18e1590288f6