build: drop the gRPC transport that google-cloud-storage never uses here

[marevol]

Why

google-cloud-storage ships two transports and declares the dependencies of both.
GcsClient and the gcs: URL handler both build their client with
StorageOptions.newBuilder(), which is the JSON-over-HTTP transport; the gRPC one is
reachable only through StorageOptions.grpc(), which nothing calls. Its stack was on the
classpath anyway, and in every artifact that depends on fess-crawler:

29 jars, 26.4 MiB removed, nothing added. grpc-xds (10.5 MiB) and
grpc-netty-shaded (10.3 MiB) are 21 MiB of that between them.

Three things deliberately kept

Each of these looks like part of the gRPC stack and is not:

Kept	Why
io.grpc:grpc-api	HttpStorageRpc#startSpan opens an OpenCensus span on every request, and OpenCensus keeps its current span in io.grpc.Context — which has lived in grpc-api since gRPC 1.60. The grpc-context artifact still exists but no longer holds the class.
proto-google-cloud-storage-v2	JsonConversions#bucketInfoDecode uses com.google.storage.v2.BucketName. The v2 protos are shared between the transports.
opentelemetry-api, -context, -common	gax-httpjson uses them.

Both of the first two were found by running the tests, not by reading the dependency
tree. A first attempt excluded io.grpc:* and proto-google-cloud-storage-v2; that
compiles clean and then throws on the first call:

java.lang.NoClassDefFoundError: io/grpc/Context
  at io.opencensus.trace.unsafe.ContextManagerImpl.currentContext
  ...
  at com.google.cloud.storage.spi.v1.HttpStorageRpc.startSpan(HttpStorageRpc.java:384)

java.lang.NoClassDefFoundError: com/google/storage/v2/BucketName
  at com.google.cloud.storage.Utils.lambda$static$4(Utils.java:155)
  at com.google.cloud.storage.JsonConversions.bucketInfoDecode(JsonConversions.java:613)

Worth knowing for anyone reviewing a similar change: compile-clean says nothing here.

Why the OpenTelemetry SDK and the generated gRPC client are safe

Not by argument — by reading the constant pools of the classes in
google-cloud-storage-2.71.0.jar:

• Exactly one class references io/opentelemetry/sdk or
  com/google/cloud/opentelemetry: OpenTelemetryBootstrappingUtils. Its only caller in
  the jar is GrpcStorageOptions.
• Every class referencing com/google/storage/v2/StorageClient is either a Grpc* class
  or a BlobWriteSessionConfig factory, which only a caller asking for gRPC uploads
  reaches.

Verification

mvn test → 2057 tests, 0 failures, 0 errors, BUILD SUCCESS, including
GcsClientTest (7 tests) against fsouza/fake-gcs-server via testcontainers, which
exercises create, doGet and doHead over the real JSON API.

Note for the Fess side

Fess declares google-cloud-storage directly too, so these exclusions do not reach it;
codelibs/fess needs the same block. That is a separate PR.