completion: CTRL+Space is not applicable in Delete/Backspace triggered session

[ivanbasov]

Both old and new completions.
Start with

class C
{
    void Method()
    {
        var s = "";
        s.Length$$
    }
}

Intellisense TriggerOnDelete should be ON
SendBackspace
SendCommitIfUnique

Expected
No active completion session
s.Length is in the current line

Actual
Active completion session
s.Lengt is in the current line

[ivanbasov]

This is a follow up for #35310. It seems there is no need to fix it in 16.1. The issue is not a regression and happened long ago.