Sorry, but the default watchOS timers (yellow pins) can not be removed, only those timers that you create can be deleted.
...

...
copied from: Set timers on Apple Watch - Apple Support
If you create your own (custom) timers and pin them on your Timers screen, the default timers will move downwards, out of sight.
However, if you use the Feedback link, you can tell Apple directly about your wishes for future watchOS updates.
This is the only way to reach them, as Apple does not follow theses user to user communities.
Feedback - Apple Watch - Apple