Remove incorrect uses of __attribute__((__malloc__)).

Remove incorrect uses of __attribute__((__malloc__)).

xrealloc() is obviously incorrect, modern GCC docs even
mention realloc() as an example where this attribute
cannot be used.

liblzma's lzma_alloc() and lzma_alloc_zero() would be
correct uses most of the time but custom allocators
may use a memory pool or otherwise hold the pointer
so aliasing issues could happen in theory.

The xstrdup() case likely was correct but I removed it anyway.
Now there are no __malloc__ attributes left in the code.
The allocations aren't in hot paths so this should make
no practical difference.