Remove broken memory-tracking crate feature

[jhorstmann]

Describe the bug

The non-default memory-check feature fails to compile (probably at least since 2021-04, that was the last time alloc/mod.rs was changed).

To Reproduce

$ cargo test --features memory-check
error[E0432]: unresolved import `crate::memory`
  --> arrow/src/zz_memory_check.rs:24:16
   |
24 |     use crate::memory::ALLOCATIONS;
   |                ^^^^^^ could not find `memory` in the crate root

Expected behavior

Since it seems to be unused, it is probably fine to remove the feature and the static ALLOCATIONS counter. If we decide to keep the feature, I would suggest to also move the increments/decrements of that counter behind the feature, as it could cause some contention in heavy multithreaded programs.

Additional context
Add any other context about the problem here.

[alamb]

I think we should remove it 👍