Stop having StaticArrays-specific `LU` and `QR` implementations

[thchr]

After https://github.com/JuliaLang/julia/pull/42594, we should in principle be able to return LinearAlgebra.LU instead of StaticArrays.LU from lu (similarly for qr). It's not immediately swappable though, since LinearAlgebra.LU stores the factors while StaticArrays currently stores the L and U matrices explicitly.

Would be nice to get rid of these, to obviate the need to include them in a "core" package (cf. https://github.com/JuliaArrays/StaticArrays.jl/issues/1023#issuecomment-1133756057).