Nav Unification: Drop support for "object of objects" adminMenu state shape

[mreishus]

Changes proposed in this Pull Request

• No functional changes

Sometimes we would get back menu specifications like:

{ "1": {MenuItem},
  "2": {MenuItem},
  "3": {MenuItem},
}

Now it's always
[ {MenuItem}, {MenuItem}, {MenuItem} ]

We already dropped support for it, but I found two extra places it can
be simplified.

Testing instructions

• Visit http://calypso.localhost:3000/plans/example.com?flags=nav-unification
• Ensure the unified sidebar continues to render and also displays children elements as appropriate (for example, Posts should expand to show sub-menu-items under)

related to #45435

[matticbot]

Test live: https://calypso.live/?branch=update/unified-sidebar-proptypes

[matticbot]

Here is how your PR affects size of JS and CSS bundles shipped to the user's browser:

Async-loaded Components (~8 bytes added 📈 [gzipped])

name                                          parsed_size           gzip_size
async-load-my-sites-sidebar-unified-switcher        +15 B  (+0.0%)       +8 B  (+0.0%)

React components that are loaded lazily, when a certain part of UI is displayed for the first time.

Legend

What is parsed and gzip size?

Parsed Size: Uncompressed size of the JS and CSS files. This much code needs to be parsed and stored in memory.
Gzip Size: Compressed size of the JS and CSS files. This much data needs to be downloaded over network.

Generated by performance advisor bot at iscalypsofastyet.com.

[obenland]

🚢 it.

[getdave]

Thanks for following up on this one @mreishus. I neglected to do so. It's testament to the rendering code that things didn't just break entirely!