Remove CHROMECAST_BUILD from chromecast/

I noticed this used by mistake in a couple CLs recently. The define
isn't set in gn, misleadingly makes people think their code is
tested on cast_shell bot.

TBR=slan@chromium.org

Bug: None
Test: None
Change-Id: Ie4b16ca4b1bb565fe8d45598f7cc0601277907d9
Reviewed-on: https://chromium-review.googlesource.com/1139214
Commit-Queue: Luke Halliwell <halliwell@chromium.org>
Reviewed-by: Luke Halliwell <halliwell@chromium.org>
Cr-Commit-Position: refs/heads/master@{#576276}
1 file changed