Remove unnecessary .getPlaintext() parameter

Already did this for `test/TestServer.scala` as part of the gRPC bump.
`test_version/version_specific_tests_dir/TestServer.scala` gets the same
treatment now to fix `test_scala_version 2.11.12` from
`test_version.sh`, which failed with:

```
ERROR: .../test_version/test_scala_version_1731027515/BUILD:161:14:
  scala @//:lib_with_scala_proto_dep failed: (Exit 1): scalac failed:
  error executing command (from target //:lib_with_scala_proto_dep)

bazel-out/.../bin/external/io_bazel_rules_scala/src/java/io/bazel/rulesscala/scalac/scalac
  @bazel-out/.../bin/lib_with_scala_proto_dep.jar-0.params

TestServer.scala:70: error: too many arguments for method usePlaintext: ()?0
    val channel = ManagedChannelBuilder.forAddress(host, port).usePlaintext(true).build
                                                                           ^
```

Author: mbland

test_version/version_specific_tests_dir/TestServer.scala

@@ -67,7 +67,7 @@ class TestServer(executionContext: ExecutionContext) { self =>
 
 object TestClient {
   def apply(host: String, port: Int): TestClient = {
-    val channel = ManagedChannelBuilder.forAddress(host, port).usePlaintext(true).build
+    val channel = ManagedChannelBuilder.forAddress(host, port).usePlaintext().build
     val blockingStub = TestServiceGrpc.blockingStub(channel)
     new TestClient(channel, blockingStub)
   }