Remove disable_nacl=1 from the nacl download script.

mojo requires nacl to build. Having the build tools silently
not download nacl causes confusion. This CL removes the early
exit if disable_nacl=1 is set in the GYP_DEFINES.

BUG=#639
R=jamesr@chromium.org

Review URL: https://codereview.chromium.org/1641673005 .
diff --git a/build/download_nacl_toolchains.py b/build/download_nacl_toolchains.py
index b99b940..4383bce 100755
--- a/build/download_nacl_toolchains.py
+++ b/build/download_nacl_toolchains.py
@@ -11,9 +11,6 @@
 
 
 def Main(args):
-  # Exit early if disable_nacl=1.
-  if 'disable_nacl=1' in os.environ.get('GYP_DEFINES', ''):
-    return 0
   script_dir = os.path.dirname(os.path.abspath(__file__))
   src_dir = os.path.dirname(script_dir)
   nacl_dir = os.path.join(src_dir, 'native_client')