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

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:
  https://github.com/openwrt/packages/commit/612c53fc6c3d9ba2a57f7329baf055f1d59a9246
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 <redacted>