sim: Drop a hack from MemPools which reset the free page.

This made it skip over 70 pages to be "what it was before" my page table
changes. I'm not sure what changes this is referring to, and the class
which manages page tables in the guest memory uses the allocPhysPages
method to allocate its memory and would cooperate with anything else
using this mechanism without having to have special accomodation.

I removed this hack and hello world seems to work fine, but there may be
some other test case which exposes some problems.

Change-Id: I16e0d8835452df9c3e79738a1eed05b4cc9372b7
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/50349
Tested-by: kokoro <noreply+kokoro@google.com>
Reviewed-by: Matthew Poremba <matthew.poremba@amd.com>
Maintainer: Gabe Black <gabe.black@gmail.com>

src/sim/mem_pool.cc

@@ -157,12 +157,6 @@ MemPools::populate(const System &sys)
 
     for (const auto &mem : memories)
         pools.emplace_back(pageShift, mem.start(), mem.end());
-
-    /*
-     * Set freePage to what it was before Gabe Black's page table changes
-     * so allocations don't trample the page table entries.
-     */
-    pools[0].setFreePage(pools[0].freePage() + 70);
 }
 
 Addr