Revert "RPC: prevent crash with walletpassphrase"

This crash seemed to have never happened in 0.8.x, and we are missing the later commit
in a8db31c83d6a43e07f741f7f61b1bf0df87621c7 so it seems better to just revert this.

This reverts commit 036a69c13c06670b46d3c02cb47e3fc7b2487854.