Remove support for avatar image formats other than PNG

Support for other formats was deemed unnecessary in the code review
and therefore removed. The value for the constant TOX_AVATARFORMAT_PNG
is now set in stone; if the other formats become needed again in the
future, this commit shall be reverted and the enum values reordered to
keep compatibility.
Remove chattiness from avatar data transfers

The chatty approach for the avatar data transfer was intended as a
security feature to add explicit delays to the transfer and prevent
amplification attacks among authenticated friends. This was deemed
unnecessary in the code review and, therefore, replaced by a simpler
approach that sends all data in a single burst.