memory plugin: Remove "slab" and "available" memory.

memory plugin: Remove "slab" and "available" memory.

Under Linux, the plugin reported "slab" (kernel data structures cache) and
"available" (estimate of how much memory might be available without swapping).
These metrics group memory along different dimensions than other reported
memory, leading to double counting of some memory. The result was that the sum
of all memory metrics was not constant.