agent: Cleanup caching code for command GET_PASSPHRASE.

* agent/command.c (cmd_get_passphrase): Read from the user cache.

We used to read the passphrase with mode CACHE_MODE_NORMAL but we put
it into the cache with CACHE_MODE_USER. However, agent_get_cache does
not yet distinguish between them and thus this does not change
anything.

(backported from commit 23d2ef83cda644c6a83499f9327350d3371e8a17)