Remove Use_lmkd_stats_log from product variables

Use_lmkd_stats_log used to control LMKD_LOG_STATS when compiling lmkd.

However, for newer Android releases, we mandate that lmkd informs statsd of process kills (change id I8cb123b9488fbc6e88863c2f0e75f1422bcd282e)
After aosp/1555318 we compile lmkd with statsd by default, so this setting is a noop.

Change-Id: Ibcf661e205b3ed2a4a520a42952aaece503e9350
Test: build
Bug: 177985094