[validation] Remove useless ret parameter from Invalid()

Summary:
ValidationState::Invalid() takes a parameter ret which is returned to
the caller. All call sites set this to false. Remove the ret parameter
and just return false always.

Partial backport of Core PR15921 : https://github.com/bitcoin/bitcoin/pull/15921/commits/7204c6434b944f6ad51b3c895837729d3aa56eea

Test Plan: ninja check check-functional

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D6885