storage_proxy: move "else if" remainder into "else" block
storage_proxy: move "else if" remainder into "else" block

In the following commit, more code will be added to the newly created
"else" block.

scylladb/scylla,avikivity/scylla,scylladb/scylla,avikivity/scylla,scylladb/scylla,avikivity/scylla,scylladb/scylla