Discard unused locals cleanup (dotnet/corefx#41876)
* Discard result from GetC14NDigest.
GetC14NDigest is being used to update the `hashAlg` from the
`GetDigestedOutput`, as well as update the `_digestedSignedInfo` and
cache flag. The return value is not needed. Discard it to make that
clearer.
* Discard the result from `CertControlStore`.
The comment above the change explains the reason for discarding
the result.
* Ignore result from CryptAcquireContext.
The comment below justifies the reason for ignoring the local.
* Code review feedback.
Commit migrated from https://github.com/dotnet/corefx/commit/
ef8756c03ff3808e2b065e6cf5c07aa169ac868f