qtbase: remove redundant open_memstream stub (relibc provides it)

The qtbase recipe's strtold_cpp_compat.c contained a 5-line stub
for open_memstream() that wrapped the call as tmpfile() with
a zero-sized allocation. This was a workaround for relibc not
implementing open_memstream.

The Round 7 follow-up added a real open_memstream implementation
in relibc at src/header/stdio/open_memstream.rs (124 lines) with
proper MemstreamWriter struct, sync_output to caller's bufp/sizep,
and a full Write trait impl. Per the project's zero-tolerance
stub policy, this stub is now redundant and must be removed.

After removal, Qt6 base links against relibc's real open_memstream
which provides proper in-memory stream semantics (vs the previous
tmpfile() workaround which gave a tmpfile with zero bytes that
was never written).

local/recipes/qt/qtbase/recipe.toml

@@ -97,15 +97,6 @@ long double relibc_compat_cpp_strtold(const char *nptr, char **endptr)
 long double relibc_compat_cpp_strtold(const char *nptr, char **endptr) {
     return strtold(nptr, endptr);
 }
-
-#include <stdio.h>
-#include <stdlib.h>
-#include <string.h>
-FILE *open_memstream(char **ptr, size_t *sizeloc) {
-    *ptr = calloc(1, 1);
-    *sizeloc = 0;
-    return tmpfile();
-}
 EOF
 "${GNU_TARGET}-gcc" \
     --sysroot="${COOKBOOK_SYSROOT}" \