refactor: Remove unused CC_TOOLCHAIN definition (#2981)

Fixes #2979.

The definition appears unused and helps advance the goal of entirely
removing current_cc_toolchain: see
https://github.com/bazelbuild/bazel/issues/26282.

python/private/attributes.bzl

diff --git a/python/private/attributes.bzl b/python/private/attributes.bzl
index c3b1cad..641fa13 100644
--- a/python/private/attributes.bzl
+++ b/python/private/attributes.bzl
@@ -156,12 +156,6 @@
         if key in kwargs
     }
 
-CC_TOOLCHAIN = {
-    # NOTE: The `cc_helper.find_cpp_toolchain()` function expects the attribute
-    # name to be this name.
-    "_cc_toolchain": attr.label(default = "@bazel_tools//tools/cpp:current_cc_toolchain"),
-}
-
 # The common "data" attribute definition.
 DATA_ATTRS = {
     # NOTE: The "flags" attribute is deprecated, but there isn't an alternative