replace_object: don't check read_replace_refs twice

replace_object: don't check read_replace_refs twice

Since e1111cef (inline lookup_replace_object() calls,
May 15 2011) the read_replace_refs global variable is
checked twice, once in lookup_replace_object() and
once again in do_lookup_replace_object().

As do_lookup_replace_object() is called only from
lookup_replace_object(), we can remove the check in
do_lookup_replace_object().

Signed-off-by: Christian Couder <redacted>
Signed-off-by: Junio C Hamano <redacted>