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.