sys: remove lib/hashtable.c

Quoting myself:

> The code of sys/lib/hashtable.c is very badly adapted for embedded
> systems. The used primes are huge, and the limit is entirely
> unrealistic. Why do we ship this file?

In #1564 we came to the conclusion that we don't need this code.

Closes #1564.
Merge pull request #1572 from Kijewski/issue-1564

sys: remove lib/hashtable.c