samples/basic/hash_map: Remove clarification sentence

This current sentence would need to be clarified a bit
further to be really correct:
It is only when building with the host C library that it
applies, but with native_sim we can build with other
libC's.
The twister tests definitions are testing only with
embedded C libraries,
and there is anyway no harm if somebody defines the heap
size even if using the host libC.
So as such it would seem better to just remove this
sentence.

Signed-off-by: Alberto Escolar Piedras <alberto.escolar.piedras@nordicsemi.no>