session: remove redundant key attribute to rpc_session_set()

session: remove redundant key attribute to rpc_session_set()

The given const char *key was used to look up the blob attribute, while
the blob attributes internal name was used to store it in the avl tree.

This leads to confusion and potential memory leaks when the given key name
does not match the blob attributes internal name.

Signed-off-by: Jo-Philipp Wich <jo@mein.io>