chore: in rules_js MODULE.bazel use default value for name of node.toolchain()

[gregmagolan]

No reason to set an explicit name here since it always has to be DEFAULT_NODE_REPOSITORY or else the following line in extensions.bzl will fail downstream:

            if toolchain.name != DEFAULT_NODE_REPOSITORY and not mod.is_root:
                fail("Only the root module may provide a name for the node toolchain.")