[MINOR][CONNECT] Remove redundant user_context.user_id assignment in execute_command methods

[ybapat]

What changes were proposed in this pull request?

Remove two redundant assignments of req.user_context.user_id = self._user_id from SparkConnectClient.execute_command and SparkConnectClient.execute_command_as_iterator.

Both methods call _execute_plan_request_with_metadata() to build the request, and that helper already sets req.user_context.user_id under the identical if self._user_id: guard. The subsequent reassignments in the two callers are dead code.

# _execute_plan_request_with_metadata() already does this:
if self._user_id:
    req.user_context.user_id = self._user_id

# These lines in execute_command / execute_command_as_iterator were redundant:
- if self._user_id:
-     req.user_context.user_id = self._user_id

Relates to GitHub issue #56408.

Why are the changes needed?

Dead code removal / readability improvement. Setting the same protobuf field to the same value twice in sequence is misleading — it implies the first assignment might not be sufficient when it is.

Does this PR introduce any user-facing change?

No. The resulting req.user_context.user_id value is identical before and after this change.

How was this patch tested?

No behavior change; existing Spark Connect unit tests cover execute_command / execute_command_as_iterator request construction.

This PR was created with the assistance of Claude (AI). Disclosed per Apache Spark contribution guidelines.

[uros-b]

Thank you @ybapat for dead code removal! cc @Yicong-Huang @HyukjinKwon