tee-supplicant: Cleanup TEEC_TEST_LOAD_PATH

Cleanup TEEC_TEST_LOAD_PATH and CFG_TA_TEST_PATH, as the
TEEC_TEST_LOAD_PATH was not working properly anyway.

TEEC_TEST_LOAD_PATH and CFG_TA_TEST_PATH are removed, and the
TEEC_LOAD_PATH is supposed to hold all paths where to search for TAs.

Signed-off-by: Mika Tammi <mika.tammi@unikie.com>
Reviewed-by: Etienne Carriere <etienne.carriere@foss.st.com>
Reviewed-by: Jens Wiklander <jens.wiklander@linaro.org>