build: remove examples/simple.rs

When `cargo test` is executed,
all examples are built by default
to ensure that they can be compiled.

This is a documented and expected behaviour,
even though it was previously reported as a bug:
<https://github.com/rust-lang/cargo/issues/6675>

In particular, `examples/simple.rs` is built into
a 67M binary `target/debug/examples/simple`.
This is unnecessary to do so every time
you change a line in the `deltachat` crate
and want to rerun the tests.

Workaround is to run `cargo test --tests`,
but it is easy to forget and is not discoverable
unless you read the "Target Selection" section of `cargo help test`.

We have a maintained example at https://github.com/deltachat-bot/echo,
so there is no need for an example in the core repository.

Cargo.toml

@@ -118,11 +118,6 @@ members = [
   "format-flowed",
 ]
 
-[[example]]
-name = "simple"
-path = "examples/simple.rs"
-
-
 [[bench]]
 name = "create_account"
 harness = false