kselftest/arm64: Remove validation of extra_context from TODO

When fixing up support for extra_context in the signal handling tests I
didn't notice that there is a TODO file in the directory which lists this
as a thing to be done. Since it's been done remove it from the list.
Signed-off-by: NMark Brown <broonie@kernel.org>
Link: https://lore.kernel.org/r/20221027110324.33802-1-broonie@kernel.orgSigned-off-by: NWill Deacon <will@kernel.org>

tools/testing/selftests/arm64/signal/testcases/TODO

 - Validate that register contents are saved and restored as expected.
-- Support and validate extra_context.