Android: Docs: Remove mention of adb shell setprop debug.assert 1

Without our new(ish) scheme of bytecode-rewriting asserts, the system
property isn't used.

NOTRY=true  # Doesn't affect bots

Change-Id: Ie0f9f2e9bb8674bba52380fc7983a13f54ae65f9
Reviewed-on: https://chromium-review.googlesource.com/692816Reviewed-by: Eric Stevenson <estevenson@chromium.org>
Commit-Queue: Andrew Grieve <agrieve@chromium.org>
Cr-Commit-Position: refs/heads/master@{#505400}

docs/android_test_instructions.md

@@ -34,12 +34,6 @@ If this option is greyed out, stay awake is probably disabled by policy. In that
 case, get another device or log in with a normal, unmanaged account (because the
 tests will break in exciting ways if stay awake is off).
 
-#### Enable Asserts
-
-```
-adb shell setprop debug.assert 1
-```
-
 #### Disable Verify Apps
 
 You may see a dialog like [this