Remove deprecated functions.

Remove deprecated functions.

Deprecated functions are redundant with previous major branches of
Monocypher, and as such don't serve any meaningful purpose.  Maintaining
old branches is cheaper anyway.

Note: this will also remove them from the manual, including on the
website when updated.  We could compensate by publishing older versions
of the manual, or we could punt on it and rely on the fact that the
tarball contain the associated manual.