Remove flag `-ddump-vt-trace`
While documenting some dump flags, I noticed that ddump-vt-trace
has no uses.
According to @monoidal, this belongs to the old vectorized, which was removed in 2018.
See this MR comment.
Since this is an undocumented debug flag, that hasn't been used for at least 3 years, I suggest we simply remove it without deprecation. As this is a dump flag, this won't be part of any production builds, IMHO.