python3: Remove HostPython3 in python3-host.mk

python3: Remove HostPython3 in python3-host.mk

HostPython3 only adds a few environment variables before running host
Python. It has only two users, Build/Compile/HostPy3RunHost and
Build/Compile/HostPy3RunTarget.

HostPython3 also accesses $(PYTHON3PATH), even though python3-host.mk
does not include python3-package.mk, where the variable is defined.

This removes HostPython3 and has its two users run host Python directly.
This also combines the environment variables of HostPython3 and the two
users into HOST_PYTHON3_VARS and PYTHON3_VARS.

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