Remove legacy maktaba#system API

[dbarnett]

We now have a maktaba#syscall API for making system calls using a builder pattern that deprecates the maktaba#system API (which has a messier cross product of options upfront).

We should get rid of the redundant legacy API for the next major version. Maybe we should show deprecation warnings for it for a while beforehand. Haven't decided yet if it's worth maktaba showing deprecation warnings for stuff like this.