Maintenance: Drop HttpStateData::handleMoreRequestBodyAvailable (#2286)

Maintenance: Drop HttpStateData::handleMoreRequestBodyAvailable (#2286)

The method is unreachable: It was supposed to be a virtual method. It is
declared as such in HttpStateData class, but it should have been
declared virtual in Client (i.e. the _parent_ class). Without that
virtual declaration in Client, the caller calls a Client method with the
same name instead. This code became unreachable in 2007 commit 5f8252d2.

The current method implementation is unusable: There are two explicit
XXXs, excessive (and unclassified) level-1 debugging, and an infinite
recursion added in the same 2007 commit 5f8252d2.

It is possible to refactor this method implementation to address known
concerns. We decided to remove this method instead because proper
refactoring is costly and is likely to result in this method removal.

The infinite recursion concern was raised by Joshua Rogers
https://joshua.hu/