Remove redundant description about virtual cores

This sentence in Overcommitting section in Compute Nodes section is
talking about the relation between the number of physical cores and
its allocation ratio, so the number of virtual cores on each VM is
unnecessary.

Fixes bug 1199925

Change-Id: I96fb0ba093bc9a4f84131848ab07245ca99050d7
Merge "Remove redundant description about virtual cores"