refs: remove unused headers

In the 'refs/' namespace, some of the included header files are not
needed, let's remove them.

Signed-off-by: Karthik Nayak <karthik.188@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

refs/debug.c

@@ -1,7 +1,6 @@
 #include "git-compat-util.h"
 #include "hex.h"
 #include "refs-internal.h"
-#include "string-list.h"
 #include "trace.h"
 
 static struct trace_key trace_refs = TRACE_KEY_INIT(REFS);

refs/files-backend.c

@@ -20,7 +20,6 @@
 #include "../dir-iterator.h"
 #include "../lockfile.h"
 #include "../object.h"
-#include "../object-file.h"
 #include "../path.h"
 #include "../dir.h"
 #include "../chdir-notify.h"

refs/reftable-backend.c

@@ -11,7 +11,6 @@
 #include "../hex.h"
 #include "../iterator.h"
 #include "../ident.h"
-#include "../lockfile.h"
 #include "../object.h"
 #include "../path.h"
 #include "../refs.h"