kselftest/arm64: Remove unneeded protype for validate_extra_context()

Nothing outside testcases.c should need to use validate_extra_context(),
remove the prototype to ensure nothing does.
Signed-off-by: Mark Brown <broonie@kernel.org>
Link: https://lore.kernel.org/r/20220829160703.874492-6-broonie@kernel.orgSigned-off-by: Catalin Marinas <catalin.marinas@arm.com>

tools/testing/selftests/arm64/signal/testcases/testcases.h

@@ -79,8 +79,6 @@ struct fake_sigframe {
 
 bool validate_reserved(ucontext_t *uc, size_t resv_sz, char **err);
 
-bool validate_extra_context(struct extra_context *extra, char **err);
-
 struct _aarch64_ctx *get_header(struct _aarch64_ctx *head, uint32_t magic,
 				size_t resv_sz, size_t *offset);