ci: travis: delete unused optee_os directory
ci: travis: delete unused optee_os directory

The optee_os cloned using "repo" is not used, because we link to the
directory checked out by the CI process instead. Delete it to save about
160 MiB of disk space on the build server.

Signed-off-by: Jerome Forissier <ced78d268b4c9a0a20a59a9feeebd2ad90da03d3@linaro.org>
Acked-by: Jens Wiklander <7706914404370d7502c27a0eff493dcf491feb51@linaro.org>

pascal-brand-st-dev/optee_os,pascal-brand-st-dev/optee_os,pascal-brand-st-dev/optee_os,pascal-brand-st-dev/optee_os,pascal-brand-st-dev/optee_os