(maint) Remove assertion that's better handled in validate
(maint) Remove assertion that's better handled in validate

Validation in the try block below handles message verification. Rely on
that rather than assertion to handle content errors. This fixes a test
failure that only appeared in debug builds where the assertion check
would trigger an exception that's supposed to be caught in the try block.

puppetlabs/cpp-pcp-client,puppetlabs/cpp-pcp-client,puppetlabs/cpp-pcp-client,puppetlabs/cpp-pcp-client