Remove Env from the contract extern functions

[leighmcculloch]

I noticed today while debugging generated contract functions that there's a warning on the Env that is included in the extern contract function signatures saying that:

warning: `extern` fn uses type `Env`, which is not FFI-safe
   --> tests/account/src/lib.rs:309:14
    |
309 |         env: soroban_sdk::Env,
    |              ^^^^^^^^^^^^^^^^ not FFI-safe
    |

We could omit the Env in this function since we can instantiate it at the start of the function in Wasm builds.

Once upon a time both the test framework and real invocations when built to Wasm went through the same function which is why the Env was a parameter, but since then the extern function now only gets used externally when built to Wasm, so the parameter is unnecessary.