Remove unused pending key exchange

[christophhagen]

session_state structs include a pending_key_exchange variable, which (in my humble opinion) is unused and can be removed. Here is my reasoning:

• It is only used within session_state.c inside functions for serialization and deserialization, and the data is never modified or read.
• The data is never accessed from any code in libsignal-protocol-c.
• While the data within pending_key_exchange is accessible through several functions in the session_state.h header, this header is not included in signal-protocol.h, which is the main way for any program to interact with the code.
• There are no tests that use pending_key_exchange, except for consistency checks of session_state, where it is manually set.

So correct me if I'm wrong, but it looks like pending_key_echange is legacy code that is no longer useful and can be removed.

[dkonigsberg]

While your assessment of the usage of pending_key_exchange is likely correct, it still remains within the libsignal-protocol-java library that this library attempts to track. As such, I'll leave this on-hold until such a time as this field is removed from that library as well.