Replace scala_register_unused_deps_toolchains call

Missed this one in third_party/test/proto earlier. Also removed
unnecessary `USE_BAZEL_VERSION` expression in
test_scala_proto_library.sh. If `USE_BAZEL_VERSION` is set, it takes
precedence to begin with, and third_party/test/proto/.bazelversion
exists now after bazel-contrib#1629.

Author: mbland

test/shell/test_scala_proto_library.sh

@@ -28,7 +28,6 @@ test_scala_proto_show_generator_exception() {
     --extra_toolchains=//test/proto/custom_generator:failing_scala_proto_toolchain
 }
 
-export USE_BAZEL_VERSION=${USE_BAZEL_VERSION:-$(cat $dir/../../.bazelversion)}
 $runner test_scala_proto_library_action_label
 $runner test_scala_proto_custom_generator
 $runner test_scala_proto_show_generator_exception

third_party/test/proto/WORKSPACE

@@ -35,7 +35,10 @@ load("@io_bazel_rules_scala//scala:scala.bzl", "scala_toolchains")
 
 scala_toolchains()
 
-register_toolchains("@io_bazel_rules_scala_toolchains//...:all")
+register_toolchains(
+    "@io_bazel_rules_scala//scala:unused_dependency_checker_error_toolchain",
+    "@io_bazel_rules_scala_toolchains//...:all",
+)
 
 load("@rules_proto//proto:repositories.bzl", "rules_proto_dependencies")
 
@@ -60,7 +63,3 @@ scala_proto_repositories()
 load("@io_bazel_rules_scala//scala_proto:toolchains.bzl", "scala_proto_register_toolchains")
 
 scala_proto_register_toolchains()
-
-load("@io_bazel_rules_scala//scala:toolchains.bzl", "scala_register_unused_deps_toolchains")
-
-scala_register_unused_deps_toolchains()