Remove Severity mechanism from mem allocator

[egavrin]

Currently, severity mechanism isn't really used, so consider removing it.

This will eliminate checks like the following:

  for (mem_try_give_memory_back_severity_t severity = MEM_TRY_GIVE_MEMORY_BACK_SEVERITY_LOW;
       severity <= MEM_TRY_GIVE_MEMORY_BACK_SEVERITY_CRITICAL;
       severity = (mem_try_give_memory_back_severity_t) (severity + 1))