prohibit to run freeMemoryIfNeed when it is slave

Situation: 
    when Slave's maxmemory and maxmemory-policy are different from Master's
    some datum are in Master but they aren't in Slave.

How:
    for consistency between master and slave. it is better to turn off slave's freememoryIfNeed.

Why::
    1] at the first time, I considered to copy master's maxmemory and maxmemory-policy setting 
    when they sync for the first time. but it can cause some troubles. because Master and Slave uses memory
    differently. 

    2] even though, they uses same maxmemory-policy, slave can perform "LRU or Expire" itself.

In conclusion, Skipping freememoryIfNeed in slave is better.