efi: drop unused xmalloc0()

Keeping this separate to make it easy to revert if it's ever needed again.

src/boot/efi/util.h

@@ -35,12 +35,6 @@ static inline void *xmalloc(size_t size) {
         return p;
 }
 
-#define xmalloc0(size)                                  \
-        ({                                              \
-                size_t _size_ = (size);                 \
-                memzero(xmalloc(_size_), _size_);       \
-        })
-
 _malloc_ _alloc_(1, 2) _returns_nonnull_ _warn_unused_result_
 static inline void *xmalloc_multiply(size_t size, size_t n) {
         assert_se(!__builtin_mul_overflow(size, n, &size));