xdiff/xmacros.h: remove unused XDL_PTRFREE

This macro was added in 3443546f6e (Use a *real* built-in diff
generator, 2006-03-24), but none of the xdiff code uses it, it uses
xdl_free() directly.

If we need its functionality again we'll use the FREE_AND_NULL() macro
added in 481df65f4f (git-compat-util: add a FREE_AND_NULL() wrapper
around free(ptr); ptr = NULL, 2017-06-15).

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

xdiff/xmacros.h

@@ -34,7 +34,6 @@
 #define XDL_ADDBITS(v,b)	((v) + ((v) >> (b)))
 #define XDL_MASKBITS(b)		((1UL << (b)) - 1)
 #define XDL_HASHLONG(v,b)	(XDL_ADDBITS((unsigned long)(v), b) & XDL_MASKBITS(b))
-#define XDL_PTRFREE(p) do { if (p) { xdl_free(p); (p) = NULL; } } while (0)
 #define XDL_LE32_PUT(p, v) \
 do { \
 	unsigned char *__p = (unsigned char *) (p); \