Hiding ledger lines?

[Geoff Horton]

I'm working on a piece that has two lines (soprano and alto) on the
same staff. The alto has a helf-rest where the soprano has music, and
because of the pitches in the soprano line, the alto rest is set on a
ledger line at middle C. I don't want it there; I want it sitting on
the e-line, so I moved it with an #'extra-offset. The ledger line is,
however, still plainly visible and unsightly. (The sample below is at
200% magnification to make it more obvious, but it's quite visible at
actual size.)

In checking the archives, I found a 2004 discussion that eventually
reached the conclusion that ledger lines aren't erasable. Is this
still the case? If so, is there another way to make this output look
better?

Source:
\version "2.8.0"

soprano = \relative c'' {
  a4 a a a
}

alto = \relative c' {
  \once \override Rest #'extra-offset = #'(0 . 1.0) r2
  f4 f
}

\paper {
  ragged-right = ##t
}

\score {
  \context Staff <<
    \context Voice = one { \voiceOne \soprano }
    \context Voice = two { \voiceTwo \alto }
    >>
}

ledger.png
Description: PNG image