Drop conv dependency

[paolobarbolini]

Drops the ancient conv dependency, which isn't necessary anymore these days.

Unfortunately this is a breaking change as the ValueInto trait appears in many places in the API https://docs.rs/imageproc/0.24.0/imageproc/index.html?search=ValueInto

[theotherphil]

Thanks! I can no longer remember why I added this dependency in the first place.

[cospectrum]

Then let's increase the version to 0.25? and safety check failed because of libsdl2-2.0-0 installation probably, I'll take a look in free time.