I believe
@IO+ is right--you can't remove that completely.
Generally, it probably won't matter, because most songs have some kind of instrumental bit before the voice starts, so the voice won't be needed right away.
In the case that the voice is needed immediately, you could offset the entire song by 1 bar in your DAW, starting all your MIDI notes or any drag/dropped audio files on bar 2. Any notes in Piapro would start on bar 2 also, which should be past the grayed-out pre-send area, if memory serves. When rendering your finished song, you would then render from bar 2 to the end, so there's no silence at the beginning of the rendered file from the pre-send area.