Unix: Remove now unnecessary I/O defines, cleanup

- `LIBC_FILEIO_ENABLED` wasn't defined anywhere, even in _other platforms_.
- `NO_NETWORK` is also never defined. It probably isn't enough anyway to
  disable network APIs in the current codebase.
- `UNIX_SOCKET_UNAVAILABLE` is never defined in this code but used by some
  other platforms, clarify that.
- `NO_STATVFS` can be removed as Android supports it since API level 19,
  which is our current min SDK level. It's also only used for
  `DirAccessUnix::get_space_left()` which is anyway overridden by
  `DirAccessJAndroid::get_space_left()` so it shouldn't make a difference.
  * Fixed documentation for `DirAccess.get_space_left()`.
- `NO_FCNTL` is likely also a remnant of early Android days, in current NDK
  r23 it seems to be available. Also cleaned up unused `fcntl.h` includes.
- `NO_ALLOCA` is never defined, and we use alloca in many places now.

scene/main/node.cpp

@@ -1466,20 +1466,10 @@ bool Node::is_greater_than(const Node *p_node) const {
 
 	ERR_FAIL_COND_V(data.depth < 0, false);
 	ERR_FAIL_COND_V(p_node->data.depth < 0, false);
-#ifdef NO_ALLOCA
-
-	Vector<int> this_stack;
-	Vector<int> that_stack;
-	this_stack.resize(data.depth);
-	that_stack.resize(p_node->data.depth);
-
-#else
 
 	int *this_stack = (int *)alloca(sizeof(int) * data.depth);
 	int *that_stack = (int *)alloca(sizeof(int) * p_node->data.depth);
 
-#endif
-
 	const Node *n = this;
 
 	int idx = data.depth - 1;