shells/44bsd-csh: remove sbrk use

[brooks]

Summary

The allocator wrappers call sbrk(0) to track heap usage. With modern
malloc implementations this is of little value and sbrk is deprecated so
remove this functionality, but continue to provide a stub implemenation
of the alloc command.

[brooks]

Compiled and minimally smoke tested on an amd64 world without sbrk support.

[mjg]

why is this port still a thing

[brooks]

I'm going to take a different path for ancient tools.

[cy]

I simply removed the Malloc, Realloc, and Free functions with direct calls to the authoritative libc versions, malloc, etc...