[stdlib] Remove redundant `UnsafePointer.store` overloads

[soraros]

This also fixes an issue where overload resolution fails to find a candidate for p.store[width=width](idx, idx) when idx: Int.

[JoeLoser]

!sync

[modularbot]

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[modularbot]

Landed in 8f47cdb263d99a1e6fed49aa4036585320748c90! Thank you for your contribution 🎉