Revert "Fix completion in Analysis Server LSP mode"

This reverts commit ec8fc01d3bc33058da1ccc2f9d1d0651d07498d4.

Reason for revert: <INSERT REASONING HERE>

Original change's description:
> Fix completion in Analysis Server LSP mode
> 
> Looks like it was initially broken in
> https://dart-review.googlesource.com/c/sdk/+/122962
> 
> Change-Id: I83b565223208fc1404c61c6971fdcc41b10fb406
> Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/125964
> Reviewed-by: Konstantin Shcheglov <scheglov@google.com>

TBR=scheglov@google.com,brianwilkerson@google.com,dantup@google.com,ariaye@google.com,iinozemtsev@google.com

Change-Id: If4a86634335c3e962b37c1e97532f05ee8b9d28d
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/125974
Reviewed-by: Ivan Inozemtsev <iinozemtsev@google.com>
Commit-Queue: Ivan Inozemtsev <iinozemtsev@google.com>