remove poke_public_key() as well as dc_key_set_from_file()

[ralphtheninja]

See #19 (comment)

Also investigate a little bit into other functions that are only used by the cmdline tool.

[ralphtheninja]

@r10s Do you know any more functions at the top of your head? If you could list them here that would be really helpful.

[r10s]

• there is dc_poke_eml_file() that is is meanwhile private to cmdline.c|rs but calls into dc_receive_imf(). dc_receive_imf() needs to be sort of public for this reason.
   
  in contrast to the poke-keys, i use this poke-eml quite frequently. it is handy to reproduce why things go wrong (eg. save a eml-message in thunderbird and replay/debug it in the cmdline tool later). probably we can remove the whole poke_spec() function.
   
  however, if these parts cause trouble, we can remove dc_poke_eml_file() for now and make dc_receive_imf() library-private. rust probably has smarter ways to test a given .eml file.

• same for dc_reset_tables() - it has some use but calls into dc_sqlite3 which is normally not allowed by the user of deltachat-core

• dc_cleanup_contacts() is also part of the cmdline tool only and calls into dc_sqlite3 - here, to be honest, i cannot remember when this was used the last time. can be removed.