don't implicitly or duplicatively add user selection in chat context (#…

…5060)

It is still added if addEnhancedContext is true to preserve the behavior
for older non-webview clients for backcompat.

fix
https://linear.app/sourcegraph/issue/SRCH-819/duplicate-context-entries-appearing-when-only-using-the-selection

fix
https://linear.app/sourcegraph/issue/CODY-1972/file-selection-is-included-as-context-even-if-the-user-deletes-it



## Test plan

Open editor. Select code. Run chat. Ensure selection is not duplicated.
Now run chat without a selection @-mention. Ensure there is no selection
used in the context.