Don't install GNU grep on FreeBSD servers

Summary:
The base system already contains GNU grep.

If a BSD grep is added in the future, and the package installs
grep as ggrep, the addition will be interesting.

Meanwhile, /usr/bin/grep vs /usr/local/bin/grep causes issue and
lack of coherence.

This commit partially reverts 50b62a8499e03.

Test Plan: no-op

Reviewers: dereckson

Reviewed By: dereckson

Differential Revision: https://devcentral.nasqueron.org/D1242