Revert "ci: Use same CXXFLAGS in bootstrap workflow"

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'.

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

This reverts Rust-GCC#1623 commit b322288.

Author: tschwinge

.github/workflows/bootstrap.yml

@@ -4,9 +4,6 @@ on:
   push:
     branches: [ master ]
 
-env:
-  CXXFLAGS: "-Wno-unused-parameter -Werror=overloaded-virtual"
-
 jobs:
   build: