sp-phragmen should only depend on sp-arithmetic

[kianenigma]

Phragmen should not use any logic aside from some arithmetic primitives. Last time I checked it depends on sp_runtime (which re-exports the arithmetic stuff) and only uses the arithmetic.

This dependency should be removed (on sp_runtime) as a step toward moving phragmen out of substrate

[kianenigma]

This can only be done after #5452

[kianenigma]

unblocked @DarkEld3r