bionic-amd64-tf-m-build: Remove outdated ArmClang 6.13

We switched to 6.18 for the last release, then to 6.21 after it.

Signed-off-by: Paul Sokolovsky <paul.sokolovsky@linaro.org>
Change-Id: I893be445beca389d632efb5620e712f910b6e873
3 files changed