Remove scalafmt_toolchain dep_providers default

It's likely that no one will ever rely on the default when defining
their own `scalafmt_toolchain`.

While investigating bazel-contrib#1675, I realized the `dep_providers` default was
set to a nonexistent target. This didn't break our tests because our
Scalafmt toolchains are created by `setup_scala_toolchain`, which sets
`dep_providers` explicitly.

I thought about aliasing the provider generated for the toolchain for
`SCALA_VERSION` in `@io_bazel_rules_scala_toolchains//scalafmt`, or
generating a new one. I ultimately decided to remove the default,
because the `deps_providers` is literally the only attribute. Anyone
defining their own `scalafmt_toolchain` will certainly define their own
`deps_providers` target(s).

Author: mbland

scala/scalafmt/toolchain/toolchain.bzl

@@ -14,12 +14,7 @@ def _scalafmt_toolchain_impl(ctx):
 scalafmt_toolchain = rule(
     _scalafmt_toolchain_impl,
     attrs = {
-        "dep_providers": attr.label_list(
-            default = [
-                "@io_bazel_rules_scala//scala/scalafmt:scalafmt_classpath_provider",
-            ],
-            providers = [_DepsInfo],
-        ),
+        "dep_providers": attr.label_list(providers = [_DepsInfo]),
     },
 )