refactor(lib/attestation): cleanup struct token_sign_cntxt

Remove fields from struct token_sign_cntxt that are managed by
runtime/rsi/realm_attest.c

Change-Id: I7adf4d6a1aaf68224cd709c749f7cca7ccacc860
Signed-off-by: Mate Toth-Pal <mate.toth-pal@arm.com>