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: Mark Brown <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Will Deacon <[email protected]>

Author: broonie

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

@@ -1,2 +1 @@
 - Validate that register contents are saved and restored as expected.
-- Support and validate extra_context.