Remove synchronization in WifiMonitorSingleton.

This isn't really required because the static initializer
for WifiMonitorSingleton won't run until the first access
to the (non primitive) static field "sInstance". Further,
static initializers will be run precisely once (i.e
first accesses to sInstance from multiple threads are safe).

Change-Id: I78fae225e4c5cb917e7e7817522595ae026daf47