Update showimage.cpp removing extra std::cerr outputs (backport #785)

[mergify]

Description

Right now when running this you get the following:

/# ros2 run image_tools showimage 
[INFO] [1777605547.094321809] [showimage]: Subscribing to topic 'image'
[INFO] [1777605547.203184824] [showimage]: Received image #camera_frame
Received image #camera_frame
[INFO] [1777605547.533933569] [showimage]: Received image #camera_frame
Received image #camera_frame
[INFO] [1777605547.538867302] [showimage]: Received image #camera_frame
Received image #camera_frame
[INFO] [1777605547.543719581] [showimage]: Received image #camera_frame
Received image #camera_frame
[INFO] [1777605547.548553095] [showimage]: Received image #camera_frame
Received image #camera_frame
[INFO] [1777605547.553280199] [showimage]: Received image #camera_frame
Received image #camera_frame

The std::cerr message is redundant and doesn't actually display any additional helpful information. I propose we remove it since it doesn't seem to serve a purpose.

Is this user-facing behavior change?

This just changes the terminal output when running showImage.

Did you use Generative AI?

No.

---

This is an automatic backport of pull request #785 done by [Mergify](https://mergify.com).

[ahcorde]

Pulls: #787
Gist: https://gist.githubusercontent.com/ahcorde/0535906ad4b8a72c11c71a22bd726c7d/raw/3d1f110db9c76bf95a012c43346b9710b4c2d703/ros2.repos
BUILD args: --packages-above-and-dependencies image_tools
TEST args: --packages-above image_tools
ROS Distro: jazzy
Job: ci_launcher
ci_launcher ran: https://ci.ros2.org/job/ci_launcher/19113

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