docs: remove SEL2 SPMC threat model

The SEL2/Hafnium SPMC implementation threat model is now hosted at [1].

[1] https://hafnium.readthedocs.io/en/latest/threat_model_spm.html

Signed-off-by: Olivier Deprez <olivier.deprez@arm.com>
Change-Id: I83d3f21ef0ee9364529c7b80de9872034ff92b09
2 files changed