config: let `config_store_data_clear()` handle `key`

config: let `config_store_data_clear()` handle `key`

Instead of remembering to free `key` in each code path, let
`config_store_data_clear()` handle that.

We still need to free it before replacing it, though. Move that freeing
closer to the replacing to be safe. Note that in that same part of the
code, we can no longer set `key` to the original pointer, but need to
`xstrdup()` it.

Signed-off-by: Martin Ågren <martin.agren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>