Android: Fix multidex proguard config messing up obfuscation

This rule was causing @MainDex methods to be obfuscated, even when they
were marked as -keep by a previous rule.

Turns out that this rule is entirely unnecessary for maintaining
@MainDex annotations anyways.

Bug: 887954
Change-Id: I3f371d4c24f91e217494f02b45e9401a8628688a
Reviewed-on: https://chromium-review.googlesource.com/1238784Reviewed-by: Sam Maier <smaier@chromium.org>
Commit-Queue: agrieve <agrieve@chromium.org>
Cr-Commit-Position: refs/heads/master@{#593300}

build/android/multidex.flags

@@ -6,9 +6,3 @@
 # can be used to create the main dex list.
 -keepattributes *Annotations*
 -keep @interface org.chromium.base.annotations.MainDex
-
-# Keep the members only if the class is kept. Also required for the annotation
-# itself to be kept, which is necessary for main dex list processing.
--keepclassmembers,allowobfuscation class * {
-  @**.MainDex *;
-}