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.