`std` contains a duplicate function for getting the gas in the current context

[Braqzen]

Summary

Two functions exist under different names in two different places. I will argue that the first one should remain (because of the explicit name that differentiates from the global gas) and the one at the bottom should be removed. Although "context_gas" may not be the best name imo.

https://github.com/FuelLabs/sway/blob/a3ffd94c413ca62a8fc130a55b296ee61d6ab1e5/sway-lib-std/src/context/registers.sw#L54-L66

https://github.com/FuelLabs/sway/blob/a3ffd94c413ca62a8fc130a55b296ee61d6ab1e5/sway-lib-std/src/context.sw#L23-L28