headers: Remove workarounds for undeffing _aligned_malloc

These workarounds stem from an earlier attempt to work around clashes
between _mm_malloc and _mm_free with _aligned_malloc, see
228f1adee765b7ff16575b816ad07745d3f2bc9b for where the earlier
"#define _aligned_malloc ..." workaround was removed.

Signed-off-by: Martin Storsjö <martin@martin.st>