\context PianoStaff <<
  \new Staff \with { \remove "Time_signature_engraver" }
  {
    <<
      \relative c{
        \clef treble
        \key c \major 
        <c e g>4
      }
    >>
  }
  \new Staff \with { \remove "Time_signature_engraver" }
  {
    <<
      \relative c {
        \clef bass
        \key c \major 
        r4
      }
    >>
  }
>>