treewide: Remove Python variants for non-Python packages

This removes Python-related build variants, and adds
PYTHON3_PKG_BUILD:=0 and minor build adjustments (where appropriate),
for non-Python packages. There should be no changes to build output.

This also updates some include paths for python3-package.mk and/or
python3-host.mk to be relative to the package Makefile.

Signed-off-by: Jeffery To <jeffery.to@gmail.com>

net/radicale2/Makefile

@@ -5,7 +5,7 @@ include $(TOPDIR)/rules.mk
 
 PKG_NAME:=radicale2
 PKG_VERSION:=2.1.11
-PKG_RELEASE:=1
+PKG_RELEASE:=2
 
 PKG_LICENSE:=GPL-3.0
 PKG_LICENSE_FILES:=COPYING
@@ -31,7 +31,6 @@ $(call Package/radicale2/Default)
   USERID:=radicale2=225:radicale2=225
   DEPENDS:=+python3 +python3-dateutil +python3-vobject +python3-setuptools
   CONFLICTS:=radicale
-  VARIANT:=python3
 endef
 
 define Package/radicale2-examples