[MSM] Simplify content structure when singleCountry is enabled

[gabrielwalt]

When creating a site with the option singleCountry=y it is expected that the country option is ignored and that the content structure doesn't contain the country level.

• /content/__appId__/us/en should be moved to /content/__appId__/en
• /content/experience-fragments/__appId__/us/en should be moved to /content/experience-fragments/__appId__/en

There might also be a number of references that contain us/en that probably need to be adjusted accordingly:

• https://github.com/adobe/aem-project-archetype/blob/b5d852c68376ee38340b0d097182c3c5cbfec849/src/main/archetype/ui.frontend.general/src/main/webpack/static/index.html#L22
• https://github.com/adobe/aem-project-archetype/blob/b5d852c68376ee38340b0d097182c3c5cbfec849/src/main/archetype/ui.frontend.general/src/main/webpack/static/index.html#L34
• https://github.com/adobe/aem-project-archetype/blob/70fafeb086996913d8bff7da3bf7fba8e94cdabd/src/main/archetype/ui.tests/test-module/specs/aem/sites.js#L21-L22
• https://github.com/adobe/aem-project-archetype/blob/02f4078bdb9803a88c26a68b792bb823aa2df202/src/main/resources/META-INF/archetype-post-generate.groovy#L166
• https://github.com/adobe/aem-project-archetype/blob/02f4078bdb9803a88c26a68b792bb823aa2df202/src/main/resources/META-INF/archetype-post-generate.groovy#L219