`Base`: bootstrap: eliminate `Array`-specific `length` methods (#57627)
Three methods of `length` are deleted.
Made possible by moving the following methods to earlier within the
bootstrapping:
* `size(a::Array)`
* `length(t::AbstractArray)`
Improves abstract return type inference of `f(x::Array) = length(a)`,
presumably because the method count is now low enough for the
world-splitting optimization to kick in.