Remove packages/i18n when it's not needed for build

When TRANSLATION_BRANCH is undefined, we're not building a website which needs translations. However because of the way many website projects integrate lego packages, by symlinking packages/ to lego/packages/, such builds are always created with the i18n plugin present.

This is not harmless: at present, if a python >=3.9 build environment is used, this plugin will corrupt all builds with PO-file garbage.

By removing the plugin prior to a deployment build, we work around this problem, while at the same time likely improve build times.