Revert "views: Assert toolkit_views=true in BUILD.gn"

This reverts commit f93bc96d.

Reason for revert: This broke the fuchsia-*.cast bots, e.g. see
https://ci.chromium.org/p/chromium/builders/ci/fuchsia-arm64-cast/60162
with this assertion firing.

Original change's description:
> views: Assert toolkit_views=true in BUILD.gn
> 
> Now that Chromecast Linux enables this, all builds that link //ui/views
> are enabling this in GN. Add an assert so that accidental dependencies
> are diagnosed by "gn gen".
> 
> Bug: 1021770
> 
> Change-Id: I681645e97f69c22ee0410c1dba95319fddef7038
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1898622
> Commit-Queue: Michael Spang <spang@chromium.org>
> Reviewed-by: Robert Liao <robliao@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#715524}

TBR=spang@chromium.org,robliao@chromium.org

Change-Id: I5f0d6bba6d10ac475ba89926abe523391695f1ba
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: 1021770
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1918654Reviewed-by: Wez <wez@chromium.org>
Commit-Queue: Wez <wez@chromium.org>
Cr-Commit-Position: refs/heads/master@{#715546}

ui/views/BUILD.gn

@@ -12,8 +12,6 @@ import("//ui/base/ui_features.gni")
 import("//ui/ozone/ozone.gni")
 import("//ui/views/features.gni")
 
-assert(toolkit_views)
-
 config("flags") {
   defines = [ "TOOLKIT_VIEWS=1" ]
 }