Makefile.am: fix warning in "make clean"

Remove invalid CLEANFILES directive, which should have been

CLEANFILES += doxygen-doc/*

but is already covered by DX_CLEANFILES. This fixes a non-fatal
error in "make clean-generic".

Fixes dd54f333f43860aaf9641873cb45c0808e9a3d8a.

Signed-off-by: Jonas Witschel <diabonas@gmx.de>
diff --git a/Makefile.am b/Makefile.am
index c6f060a..e156be0 100644
--- a/Makefile.am
+++ b/Makefile.am
@@ -139,7 +139,6 @@
     doxygen-doc/man/Esys_ZGen_2Phase.3
 $(DOXYMAN3): doxygen-doc
 EXTRA_DIST += $(DOXYMAN3)
-CLEANFILES += doxygen-doc
 else #DOXYMAN
 DOXYMAN3 =
 endif #DOXYMAN