Remove the RDMA sidecar prototype and its mount client (#11119)

* rdma: drop the sidecar prototype

The Rust engine under it never touched a wire: rdma.rs fabricates pattern
bytes and the crate's default feature is mock-ucx, with real-ucx unimplemented
since the directory landed. Nothing builds it, no CI runs it, and its only
consumer is weed mount's RDMA client, removed next. Two 22MB binaries were
committed along with it.

Claude-Session: https://claude.ai/code/session_01X3zhqLYwQwEQCrRbuzQKvy

* mount: remove the RDMA client that spoke to the deleted sidecar

Its only server was the sidecar's HTTP API, and the path could never have
worked in production anyway: it served a single chunk per call, ignored the
buffer's chunk boundaries, and had no test. Removing it also removes the
per-handle cumulative-offset cache, which nothing else used.

The -rdma.* mount flags go with it. They defaulted to off and pointed at an
address no released build ever listened on.

Claude-Session: https://claude.ai/code/session_01X3zhqLYwQwEQCrRbuzQKvy

.gitignore

@@ -116,7 +116,6 @@ test/s3/versioning/weed-test.log
 /docker/admin_integration/data
 docker/agent_pub_record
 docker/admin_integration/weed-local
-/seaweedfs-rdma-sidecar/bin
 /test/s3/encryption/filerldb2
 /test/s3/sse/filerldb2
 test/s3/sse/weed-test.log