Revert "Build non-official app_shell installer on Linux"

This reverts commit ebc4d82a.

Reason for revert: Broke Linux Builder:
https://ci.chromium.org/p/chromium/builders/luci.chromium.ci/Linux%20Builder/99438

Original change's description:
> Build non-official app_shell installer on Linux
> 
> This causes the app_shell .deb installer to be built on non-ASAN/MSAN
> Linux CQ and waterfall bots, so regressions that break the installer
> build can be caught before hitting the official builders.
> 
> The Chrome installer was enabled for non-official Linux builders in
> https://bugs.chromium.org/p/chromium/issues/detail?id=758654#c32.
> 
> Bug: 714465
> Change-Id: Ifb6908b756d9ffc292fc131396c424d66f841a2a
> Reviewed-on: https://chromium-review.googlesource.com/956756
> Reviewed-by: Michael Moss <mmoss@chromium.org>
> Commit-Queue: Michael Giuffrida <michaelpg@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#542382}

TBR=mmoss@chromium.org,michaelpg@chromium.org

Change-Id: Id8587cea5bfcc646b0d4c2a6f2fbd52ee11904be
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: 714465
Reviewed-on: https://chromium-review.googlesource.com/958284Reviewed-by: Thomas Anderson <thomasanderson@chromium.org>
Commit-Queue: Thomas Anderson <thomasanderson@chromium.org>
Cr-Commit-Position: refs/heads/master@{#542395}

extensions/shell/installer/BUILD.gn

@@ -10,8 +10,9 @@ import("//extensions/buildflags/buildflags.gni")
 assert(enable_extensions)
 
 declare_args() {
-  enable_app_shell_installer = is_desktop_linux && current_cpu == "x64" &&
-                               !is_component_build && !using_sanitizer
+  enable_app_shell_installer =
+      is_desktop_linux && is_chrome_branded && current_cpu == "x64" &&
+      !is_component_build && !using_sanitizer
 }
 
 # Meta-target that forwards to the installer of the correct type (if any).