Discard unused locals cleanup (dotnet/corefx#41876)

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