base/complex.jl: remove pointless promotion rule.

I'm pretty sure that there's no way to actually invoke this rule.
slightly improved implementation of mean

- regionsize is moved to reducedim.jl
- avoid a temporary allocation in mean(x, region)
Merge pull request #5309 from lindahua/dh/mean2

slightly improved implementation of mean
Merge branch 'master' of github.com:JuliaLang/julia