vad : remove shortform for --vad option in cli.cpp (#3145)
This commit removes the shortform for the --vad option in cli.cpp.
The motivation for this is that `-v` is often used for verbose or
version is many tools and this might cause confusion.
Refs: https://github.com/ggml-org/whisper.cpp/pull/3065#issuecomment-2873243334
