build: remove -Qunused-arguments workaround for clang + ccache

This was added in 386efb7695debaf5f0f328a2e4c9abc342161665 to
address spammy Clang warnings when building with ccache.

The issue was addressed in ccache 3.2
(https://bugzilla.samba.org/show_bug.cgi?id=8118, Nov 2014),
and from a look at all major distros, it's only Debian Jessie
that has a version of ccache older than that (3.1).

Therefore I think it's acceptable to drop this workaround, and
re-enable warnings for unused driver arguments (when compiling
using Clang).

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