buildroot

cairo: drop CAIRO_CONF_ENV variables

These are completely historic from the initial commit (3a0a8791) and
completely unused.

Two separate builds with/without give nothing relevant with:

$ diff -u cairo.cv/config.log cairo-1.14.4/config.log|grep ^+

Same applies to removals with:

$ diff -u cairo.cv/config.log cairo-1.14.4/config.log|grep ^-

Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>