[analysis_server] Remove inTestMode from bulk fix API

This flag appears to be unused in the server. It's only used inside the `dart fix` command itself.

Since I haven't implemented an equivalent in LSP, I'd like to remove it first to verify it's definitely not being used by any code, and to avoid it looking like a change in an upcoming LSP migration CL.

Change-Id: I38ae374e08b1972b7db2fd50f2f2714e610d2cd0
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/544740
Reviewed-by: Keerti Parthasarathy <keertip@google.com>
Commit-Queue: Keerti Parthasarathy <keertip@google.com>
Reviewed-by: Brian Wilkerson <brianwilkerson@google.com>