`msg_amount()` is a duplication of `balance()`

[Braqzen]

It just calls the function

https://github.com/FuelLabs/sway/blob/e70cb773f873914ec3c9bc24bf82aa67e8af900a/sway-lib-std/src/context.sw#L75-L77

but it has a friendlier name.

We ought to only have 1 implementation.

[Braqzen]

Remove balance() in favour of msg_amount()