Remove FreeBSD CMake file (#2247)

FreeBSD has libexecinfo as a system library.

cmake/os/FreeBSD.cmake

-
-# Copyright (c) 2010, Oracle and/or its affiliates. All rights reserved.
-# 
-# This program is free software; you can redistribute it and/or modify
-# it under the terms of the GNU General Public License as published by
-# the Free Software Foundation; version 2 of the License.
-#
-# This program is distributed in the hope that it will be useful,
-# but WITHOUT ANY WARRANTY; without even the implied warranty of
-# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
-# GNU General Public License for more details.
-#
-# You should have received a copy of the GNU General Public License
-# along with this program; if not, write to the Free Software
-# Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, MA  02110-1335  USA 
-
-# This file includes FreeBSD specific options and quirks, related to system checks
-
-# Should not be needed any more, but kept for easy resurrection if needed
-#   #Legacy option, maybe not needed anymore , taken as is from autotools build
-#   ADD_DEFINITIONS(-DNET_RETRY_COUNT=1000000)
-
-# The below was used for really old versions of FreeBSD, roughly: before 5.1.9
-# ADD_DEFINITIONS(-DHAVE_BROKEN_REALPATH)
-
-# Find libexecinfo (library that contains backtrace_symbols etc)
-SET(EXECINFO_ROOT /usr/local CACHE INTERNAL "Where to find execinfo library and header")
-INCLUDE_DIRECTORIES(${EXECINFO_ROOT}/include)
-SET(CMAKE_REQUIRED_INCLUDES ${CMAKE_REQUIRED_INCLUDES} ${EXECINFO_ROOT}/include)
-SET(ENV{LIB} "$ENV{LIB}:${EXECINFO_ROOT}/lib")