.github: Remove SignedCapsulePkg

Remove SignedCapsulePkg from the following GitHub configuration files:

- ISSUE_TEMPLATE/bug_report.yml: Remove SignedCapsulePkg from the
  affected package drop-down list.
- ISSUE_TEMPLATE/feature_request.yml: Remove SignedCapsulePkg from the
  affected package drop-down list.
- advanced-issue-labeler.yml: Remove the package:signedcapsulepkg
  label rule and its associated key.

SignedCapsulePkg is no longer used. The features provided by this
package have been superseded by FmpDevicePkg.

Signed-off-by: Michael D Kinney <michael.d.kinney@intel.com>