[Chromecast] Remove is_cast_using_cma_backend flag.

All internal builds have this set to true, so no need to keep
this flag.

Merge-With: eureka-internal/288022

Bug: Internal b/136084695
Test: build/run Android TV and Android Things apks and unittests
Change-Id: I7815df32f721bf1e3c1be10a807a0fe557e4238f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1678549
Reviewed-by: Luke Halliwell <halliwell@chromium.org>
Reviewed-by: Dirk Pranke <dpranke@chromium.org>
Reviewed-by: Kenneth MacKay <kmackay@chromium.org>
Commit-Queue: Simeon Anfinrud <sanfin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#673622}

diff --git a/chromecast/chromecast.gni b/chromecast/chromecast.gni
index 8410c94e..5fca09e 100644
--- a/chromecast/chromecast.gni
+++ b/chromecast/chromecast.gni

@@ -152,7 +152,7 @@
 # Use Chromecast CDMs for protected content. Some Android platforms use
 # MediaDrm for CDM support.
 declare_args() {
-  use_chromecast_cdms = is_cast_using_cma_backend
+  use_chromecast_cdms = true
 }
 
 foreach(target_type,