remove unnecessary bytestring call for ccall string arguments

[jakebolewski]

Due to #5677, bytestring() is now called for for String type arguments in ccall. This removes the remaining explicit bytestring calls in Base for consistency.

[tkelman]

There's also the rather outdated getenv example in https://github.com/JuliaLang/julia/blob/master/doc/manual/calling-c-and-fortran-code.rst, and these could probably be simplified now https://github.com/JuliaLang/julia/blob/master/base/stat.jl#L51-L54

[jakebolewski]

I hope you were not already working on this :)

[tkelman]

nah, just searched through for places it was being used