libteec: drop benchmark framework support

Drop Benchmark Framework as current implementation is non-function
and obsolete, and it's not supported anymore.

Acked-by: Jens Wiklander <jens.wiklander@linaro.org>
Acked-by: Etienne Carriere <etienne.carriere@foss.st.com>
Signed-off-by: Igor Opaniuk <igor.opaniuk@foundries.io>