contrib/bc: remove files ommitted from the release

A number of files have been removed from the release distribution of
this bc implementation. They were mostly relevant for pre release
testing and benchmarking to identify regressions. The Markdown
sources of the man pages are only relevant for combinations of build
options not used in FreeBSD and need non-default conversion tools
(available as ports in FreeBSD).

All the omitted files can be found in the upstream git repository,
and they are fetched when building this software as a port. But they
have never been used in the FreeBSD base system.