remove ref output for examples

The examples output can vary numerically, and although we have
started marking those sections with IGNORE_RDIFF macros, there may
still be cases that are flagged by CRAN checks. Any diagnostics in
CRAN can be fatal (or tolerated: you never know and there is no
guarantee), so play safe and remove the reference output. NB, the
examples are still run in CRAN tests, but only errors are
detected.