config: Remove redundant explicit setting of default clocks

This patch removes the explicit setting of the clock period for
certain instances of CoherentBus, NonCoherentBus and IOCache where the
specified clock is same as the default value of the system clock. As
all the values used are the defaults, there are no performance
changes. There are similar cases where the toL2Bus is set to use the
parent CPU clock which is already the default behaviour.

The main motivation for these simplifications is to ease the
introduction of clock domains.

src/mem/ruby/system/RubySystem.py

@@ -36,7 +36,6 @@ class RubySystem(ClockedObject):
     random_seed = Param.Int(1234, "random seed used by the simulation");
     randomization = Param.Bool(False,
         "insert random delays on message enqueue times");
-    clock = '1GHz'
     block_size_bytes = Param.UInt32(64,
         "default cache block size; must be a power of two");
     mem_size = Param.MemorySize("total memory size of the system");