readme: Remove refs to non-existent options
readme: Remove refs to non-existent options

Options waffle_default_platform and waffle_default_gl_api were removed
a long time ago.

Signed-off-by: Chad Versace <a557f7f75bf28dc19ac99107f5cc7cd6b3fbdb1a@linux.intel.com>

waffle-gl/waffle,fjhenigman/waffle,waffle-gl/waffle,janesma/waffle,chadversary/waffle,waffle-gl/waffle,evelikov/waffle,maurossi/waffle,yinquan529/waffle-gl-waffle,yinquan529/waffle-gl-waffle,dcbaker/waffle,dcbaker/waffle,janesma/waffle,chadversary/waffle,fjhenigman/waffle,fjhenigman/waffle,evelikov/waffle,dcbaker/waffle,chadversary/waffle,janesma/waffle,yinquan529/waffle-gl-waffle,maurossi/waffle,evelikov/waffle,maurossi/waffle,waffle-gl/waffle