Remove drop reference

[davidbrochart]

The drop(self) issues a warning:

warning: calls to `std::mem::drop` with a reference instead of an owned value does nothing
   --> src/y_transaction.rs:263:9
    |
263 |         drop(self);
    |         ^^^^^----^
    |              |
    |              argument has type `&mut YTransaction`
    |
    = note: use `let _ = ...` to ignore the expression or result
    = note: `#[warn(dropping_references)]` on by default

[davidbrochart]

I also tested the change with the following code, which doesn't seem to create any memory leak:

from y_py import YDoc

doc = YDoc()
text = doc.get_text("name")
t = "hello"
n = len(t)

while True:
    with doc.begin_transaction() as txn:
        text.insert(txn, 0, t)

    with doc.begin_transaction() as txn:
        text.delete_range(txn, 0, n)