Fix #1294.

Fix #1294.

Since commit d3ee0333e179bf7b55c6941bfebcbed2a477b493, a generic "serif"
font family was added as the fallback font family for "Century
Schoolbook L".

This should work, in theory, since a PangoFontDescription permits it,
but it doesn't work very well for font selection within LilyPond.
Removing the explicit fallback font family generally works better, since
Fontconfig can choose an appropriate fallback automatically.