Make distclean issue

There is an issue when using make distclean when the code was configured with --with-cooling=EAGLE.

./configure --with-cooling=EAGLE --with-chemistry=EAGLE
make
make distclean

leads to

...
Making distclean in examples/Cooling/CoolingRates
make[1]: Entering directory '/net/winkel/data2/vandenbroucke/swiftsim/examples/Cooling/CoolingRates'
 rm -f cooling_rates
rm -rf .libs _libs
rm -f *.o
rm -f *.lo
rm -f *.tab.c
test -z "" || rm -f 
test . = "." || test -z "" || rm -f 
rm -f TAGS ID GTAGS GRTAGS GSYMS GPATH tags
rm -f ./.deps/cooling_rates-cooling_rates.Po
rm -f Makefile
make[1]: Leaving directory '/net/winkel/data2/vandenbroucke/swiftsim/examples/Cooling/CoolingRates'
Making distclean in examples/Cooling/CoolingRates
make[1]: Entering directory '/net/winkel/data2/vandenbroucke/swiftsim/examples/Cooling/CoolingRates'
make[1]: *** No rule to make target 'distclean'.  Stop.
make[1]: Leaving directory '/net/winkel/data2/vandenbroucke/swiftsim/examples/Cooling/CoolingRates'
make: *** [Makefile:825: distclean-recursive] Error 1

The reason is that examples/Cooling/CoolingRates is added twice to DIST_SUBDIRS: once as part of SUBDIRS and once explicitly. Removing one of them, as done by this MR, fixes the issue. Not sure that is the cleanest approach though.