swtpm: Remove unnecessary fields from symmetric key structure

swtpm: Remove unnecessary fields from symmetric key structure

Remove the valid field from the symmtric key structure and
user userKeyLength > 0 instead, which is the same.

Also remove the tag and fill fields, which were originally used in
TPM 1.2.

Use function calls to test whether the file or migration key is
available where possible.

Signed-off-by: Stefan Berger <stefanb@linux.vnet.ibm.com>