Remove slab size special case for growth factor 2. bug56

Using any growth factor, the minimum slab size is always 104 bytes
(unless using -n option to use a specific value) except for 2. In this
case, it's 128 bytes and cannot be reduced using -n option (it can be
increased however).

From slabs.c:
    /* Factor of 2.0 means use the default memcached behavior */
    if (factor == 2.0 && size < 128)
        size = 128;

What I sugget is to simply remove this part. If the default behaviour
needs to be kept to 128 bytes per default when factor is 128, maybe
default value could for the -n option could be adapted to do so. But I
don't see a good reason for this specific difference between factor 2
and others.

slabs.c

@@ -100,10 +100,6 @@ void slabs_init(const size_t limit, const double factor, const bool prealloc) {
     int i = POWER_SMALLEST - 1;
     unsigned int size = sizeof(item) + settings.chunk_size;
 
-    /* Factor of 2.0 means use the default memcached behavior */
-    if (factor == 2.0 && size < 128)
-        size = 128;
-
     mem_limit = limit;
 
     if (prealloc) {