build: remove centos-7 directive from Makefile
build: remove centos-7 directive from Makefile

- CentOS-7 support has been deprecated.

Type: fix

Signed-off-by: Dave Wallace <0ed937c0e4b2f09f7bc24acf16cee52b8a199d5f@gmail.com>
Change-Id: If7f1988487d0b63a596dfee9dd03af89fb159573

chrisy/vpp,FDio/vpp,chrisy/vpp,FDio/vpp,chrisy/vpp,FDio/vpp,FDio/vpp,FDio/vpp,chrisy/vpp,FDio/vpp,FDio/vpp,FDio/vpp,chrisy/vpp,chrisy/vpp,chrisy/vpp,chrisy/vpp