Remove expired flags: enable-desktop-pwas and enable-experimental-app-banners

This CL removes two flags:
 - enable-desktop-pwas: This is on by default for desktop.
 - enable-experimental-app-banners: This is on by default for Android

These flags are being removed together because
AppBannerManager::IsExperimentalAppBannersEnabled() was true if either
one was true. This was in effect true for all platforms by default.

TBR: vakh@chromium.org
Bug: 959086
Change-Id: I0c9f170f3be7ba1cacc7750c9268316912c98ff4
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1609744
Commit-Queue: Alan Cutter <alancutter@chromium.org>
Reviewed-by: Ben Wells <benwells@chromium.org>
Reviewed-by: Evan Stade <estade@chromium.org>
Reviewed-by: Azeem Arshad <azeemarshad@chromium.org>
Reviewed-by: Adrienne Porter Felt <felt@chromium.org>
Reviewed-by: Avi Drissman <avi@chromium.org>
Reviewed-by: Dominick Ng <dominickn@chromium.org>
Cr-Commit-Position: refs/heads/master@{#662060}
83 files changed