hadrian: Drop -Werror=unused-but-set-variable from GHC flags
Previously hadrian would pass -optc-Werror=unused-but-set-variable
to all GHC invocations. This was a difference from the make build system
and cause the unregisterised build to fail as the C that GHC produces
contains many unused functions. Drop it from the GHC flags.
Note, however, that the flag is still present in
Settings.Builders.Common.cWarnings and therefore will still be applied
during compilation of C sources.