[BUG] Discover datasource selector X should be removed

[seanneumann]

Describe the bug

When selecting a datasource in the Discover experience, the X button does not clear out the input (by design). So we should remove the X as an option.

To Reproduce
Steps to reproduce the behavior:

1. Go to Discover
2. Click on datasource selector
3. Click the X
4. Notice how it doesn't clear the field.

Expected behavior
Remove the X because we actually don't want it to be cleared out and the X implies that behavior.

Dashboards Version
2.10, 2.11 early build