\version "2.18.2"  % necessary for upgrading to future LilyPond versions.           
\score {
    \new Staff
    \new Voice      
    \relative c'{
        \set Staff.midiInstrument = #"acoustic guitar (string)"
        % Music notation goes here.
    }     
    \layout { }
    \midi { 
      \tempo 4 = 126
      \context {
        \Staff
        \remove "Staff_performer"
      }
      \context {
        \Voice
        \consists "Staff_performer"
      }  
    }
}