Remove double-colon from clean target which causes breakage in reverse-depends.

Index: petsc/lib/petsc/conf/rules
===================================================================
--- petsc.orig/lib/petsc/conf/rules	2021-01-17 10:48:57.039545954 +1100
+++ petsc/lib/petsc/conf/rules	2021-01-17 10:48:57.031543913 +1100
@@ -69,7 +69,7 @@
               *.class *.ouit *.ad.* g_* silly.cmp *.tmp.* *.ilk *.pdb *.inst.c *.rej *.gcda *.gcno
 	@-${RM} -rf ${CLEANDIRS} *.dSYM AD_cache SunWS_cache
 
-clean:: clean-legacy
+clean: clean-legacy
 
 #
 #  Checks if directory requires particular package or language
Index: petsc/makefile
===================================================================
--- petsc.orig/makefile	2021-01-17 10:48:57.039545954 +1100
+++ petsc/makefile	2021-01-17 10:48:57.031543913 +1100
@@ -276,7 +276,7 @@
 allclean:
 	-@${OMAKE} -f gmakefile clean
 
-clean:: allclean
+clean: allclean
 
 distclean: chk_petscdir
 	@if [ -f ${PETSC_DIR}/${PETSC_ARCH}/lib/petsc/conf/reconfigure-${PETSC_ARCH}.py ]; then \