Remove duplicate functions from conversions library

[SwayStar123]

Description

Partially closes #5797

There are implementations such as

impl u16 {
    pub fn try_as_u8(self) -> Option<u8> {
        if self <= u8::max().as_u16() {
            Some(asm(input: self) {
                input: u8
            })
        } else {
            None
        }
    }
}

which serve to attempt conversion of a u16 to a u8, however, there is already an implementation of

impl TryFrom<u16> for u8 {
    fn try_from(u: u16) -> Option<Self> {
        if u > u8::max().as_u16() {
            None
        } else {
            Some(asm(r1: u) {
                r1: u8
            })
        }
    }
}

which serves the same purpose, and with the generic auto implementation of TryInto via

impl<T, U> TryInto<U> for T
where
    U: TryFrom<T>,
{
    fn try_into(self) -> Option<U> {
        U::try_from(self)
    }
}

renders the try_as_* functions redundant.

Checklist

• I have linked to any relevant issues.
• I have commented my code, particularly in hard-to-understand areas.
• I have updated the documentation where relevant (API docs, the reference, and the Sway book).
• If my change requires substantial documentation changes, I have requested support from the DevRel team
• I have added tests that prove my fix is effective or that my feature works.
• I have added (or requested a maintainer to add) the necessary Breaking* or New Feature labels where relevant.
• I have done my best to ensure that my PR adheres to the Fuel Labs Code Review Standards.
• I have requested a review from the relevant team or maintainers.

[SwayStar123]

Blocked by #5883