Revert "ci: Apply warnings in CI"

As discussed in
<https://inbox.sourceware.org/gcc-patches/[email protected]>:

> '-Werror=overloaded-virtual' is implied as by default, we have
> '-Woverloaded-virtual' and '-Werror' [for '--enable-bootstrap' builds].
> ([...])
> (Maybe that isn't active for '--disable-bootstrap' builds, but that's
> "OK".)

For '--disable-bootstrap' builds, we still have '-Woverloaded-virtual', and any
new such diagnostics will be caught by the #1026 "check for new warnings"
machinery, so effectively that does implement '-Werror', too.

> '-Wno-unused-parameter' [did] move into
> 'rust-warn'

This reverts #1585 commit a32ef7a.

.github/workflows/ccpp.yml

@@ -8,9 +8,6 @@ on:
   pull_request:
     branches: [ master ]
 
-env:
-  CXXFLAGS: "-Wno-unused-parameter -Werror=overloaded-virtual"
-
 jobs:
   build-and-check-ubuntu-64bit: