Remove libselinux from the apex_available whitelist

libselinux no longer is included in any APEX. Only the platform variant
of it (/system/lib/libselinux.so) exists and APEXes link to it.

Removing the lib name from the whitelist to make it clear that the
library is not available to any APEX.

Bug: 151053366
Bug: 150999716
Test: m
Change-Id: Id4fb933141ad32ff5217a58f1c7d689cc657e9ea