Stop truncating .MAKEOVERRIDES after the FLAVORS work.

There is no such variable as ${MAKEOVERRIDES}, only ${.MAKEOVERRIDES}, so this
code was simply truncating .MAKEOVERRIDES entirely which could cause performance
and other odd issues with OSREL and make recursion.

We don't want to leak FLAVOR to child sub-makes but that can be fixed later;
this code was not doing what it intended.

Approved by: portmgr (implicit)
MFH: 2018Q1