python,python3: remove --ignore-installed flag for host packages

This was copied over from python-packages, when support for installing
packages host-side (via pip) was added.

Based on the discussion on this commit:
  612c53fc6c
it was mentioned that removing this may add more benefit in terms of
reducing build time, because packages won't get reinstalled every time.

I'm not entirely sure about any potential side-effects of this, but it's
worth trying it out.

Signed-off-by: Alexandru Ardelean <ardeleanalex@gmail.com>

lang/python/python-host.mk

@@ -76,7 +76,6 @@ define host_python_pip_install
 	$(HOST_PYTHON_PIP) install \
 		--root=$(1) \
 		--prefix=$(2) \
-		--ignore-installed \
 		$(3)
 endef
 

lang/python/python3-host.mk

@@ -76,7 +76,6 @@ define host_python3_pip_install
 	$(HOST_PYTHON3_PIP) install \
 		--root=$(1) \
 		--prefix=$(2) \
-		--ignore-installed \
 		$(3)
 endef