Remove error shown when omitting changed packages from release spec

[mcmire]

In order to support partial releases of a monorepo — that is, the ability to publish a subset of publishable packages within a release — we need to allow users to omit packages from the release spec that have changes since their previous release.

To do this, we need to:

• Remove the check that runs when a version is not specified for a package that is checked
• Remove the error that is thrown when that happens
• Remove intentionally-skip as a possible version specifier for the release spec

Part of MetaMask/core#1539.

[cryptodev-2s]

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