Don't pre-populate the BitBucketCredentials block's URL field

[discdiver]

First check

• [X] I added a descriptive title to this issue.
• [X] I used the GitHub search to find a similar issue and didn't find it.
• [X] I refreshed the page and this issue still occurred.
• [X] I checked if this issue was specific to the browser I was using by testing with a different browser.

Bug summary

This field is pre-populated, when most users don't need a value. I don't think a suggested greyed-out value is necessary.

Reproduction

Go to the page to create a BitBucket Credentials block in the UI.

Error

Browsers

• [X] Chrome
• [ ] Firefox
• [ ] Safari
• [ ] Edge

Prefect version

No response

Additional context

No response