gn: Remove setting |use_default_render_theme|.
gn: Remove setting |use_default_render_theme|.

|use_default_render_theme| is already set by //build/config/ui.gni, so remove
the definition from blink's config.gni

BUG=488576

Review URL: https://codereview.chromium.org/1148193009

git-svn-id: bf5cd6ccde378db821296732a091cfbcf5285fbd@196546 bbb929c8-8fbe-4397-9dbb-9b2b20218538

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