samples/net: civetweb: Remove SOC_SERIES_SAME70=y in prj.cnf

For some reasons SOC_SERIES_SAME70 was selected in civetweb sample,
which prevents build on other platforms.
This issue was silent since sample yaml file limits sample testing on
sam_e70_xplained.

Signed-off-by: Erwan Gouriou <erwan.gouriou@linaro.org>