mergetool--lib: remove no-op assignment to $status from setup_user_tool

mergetool--lib: remove no-op assignment to $status from setup_user_tool

Even though setup_user_tool assigns the exit status from "eval
$merge_tool_cmd" to $status, the variable is overwritten by the
function it calls next, check_unchanged, without ever getting looked
at by anybody.  And "return $status" at the end of this function
returns the value check_unchanged assigned to it (which is the same
as the value the function returns).  Which makes the assignment a
no-op.

Remove it.

Signed-off-by: Junio C Hamano <gitster@pobox.com>