Fixed lint warning on vs_toolchain.py.

This CL fixes the following warning:

W: 133, 2: Unused variable 'version' (unused-variable)
Change-Id: I73a3eb3fe40b46196a2fbf014c398bd90e693ced
Reviewed-on: https://chromium-review.googlesource.com/c/1317009Reviewed-by: Takuto Ikuta <tikuta@chromium.org>
Commit-Queue: Yoshisato Yanagisawa <yyanagisawa@chromium.org>
Cr-Commit-Position: refs/heads/master@{#605260}

build/vs_toolchain.py

@@ -130,7 +130,6 @@ def DetectVisualStudioPath():
     raise Exception(('Visual Studio version %s (from GYP_MSVS_VERSION)'
                      ' not supported. Supported versions are: %s') % (
                        version_as_year, ', '.join(year_to_version.keys())))
-  version = year_to_version[version_as_year]
   if version_as_year == '2017':
     # The VC++ 2017 install location needs to be located using COM instead of
     # the registry. For details see: