treewide: remove COMMITCOUNT

treewide: remove COMMITCOUNT

Automatically compute and substitute current values for all
$(COMMITCOUNT) instances as this feature is deprecated and shouldn't be
used.

Based on commit 0c10c224be81:

Change COMMITCOUNT in rules.mk to:
```
COMMITCOUNT = $(if $(DUMP),0,$(shell sed -i "s/\$$(COMMITCOUNT)/$(call commitcount)/" $(CURDIR)/Makefile))
```

then update all affected packages by:
```
for i in $(git -C feeds/packages grep -l COMMITCOUNT | sed 's^.*/\([^/]*\)/Makefile^\1^';);
do
	make package/$i/clean
done
```

Signed-off-by: Kuan-Yi Li <kyli@abysm.org>