Remove legacy vs_toolchain.py environment variable.

As of https://boringssl-review.googlesource.com/c/boringssl/+/45644, all
the builders that need vs2017 specify it via gclient_vars.

Change-Id: I9f1f4d1371ef601b525f4dd3a2a9b89490aa3171
Reviewed-on: https://boringssl-review.googlesource.com/c/boringssl/+/45804
Reviewed-by: Adam Langley <agl@google.com>

util/bot/DEPS

diff --git a/util/bot/DEPS b/util/bot/DEPS
index e710c0f..bad720f 100644
--- a/util/bot/DEPS
+++ b/util/bot/DEPS
@@ -19,7 +19,7 @@
   'checkout_sde': False,
   'checkout_nasm': False,
   'checkout_libcxx': False,
-  'vs_version': 'env',
+  'vs_version': '2015',
 
   # Run the following command to see the latest builds in CIPD:
   #  cipd describe PACKAGE_NAME -version latest

util/bot/vs_toolchain.py

diff --git a/util/bot/vs_toolchain.py b/util/bot/vs_toolchain.py
index 904f3ca..954b670 100644
--- a/util/bot/vs_toolchain.py
+++ b/util/bot/vs_toolchain.py
@@ -77,10 +77,6 @@
   information required to pass to vs_env.py which we use in
   |SetEnvironmentForCPU()|.
   """
-  # TODO(davidben): Once the builders specify the toolchain version directly,
-  # remove this and replace the default in DEPS from 'env' to '2015'.
-  if version == 'env':
-    version = os.environ.get('GYP_MSVS_VERSION', '2015')
   depot_tools_path = FindDepotTools()
   get_toolchain_args = [
       sys.executable,