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

This was added in 386efb7695 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).

configure.ac

@@ -1410,9 +1410,6 @@ if test "x$use_ccache" != "xno"; then
   fi
   AC_MSG_RESULT($use_ccache)
 fi
-if test "x$use_ccache" = "xyes"; then
-    AX_CHECK_PREPROC_FLAG([-Qunused-arguments],[CPPFLAGS="-Qunused-arguments $CPPFLAGS"])
-fi
 
 dnl enable wallet
 AC_MSG_CHECKING([if wallet should be enabled])