Revert mobile RN testsuite integration

[hypest]

Description

Removes the native mobile testsuite integration.

With the Gutenberg rollout around the corner, people are busy and the native mobile testsuite integration has been left unattended and without evolution. Not many people feel comfortable touching the related feature anyway, and that makes finding resources to work on it even harder.

As a stop gap solution until we have a better integration and coordination between the stakeholders, this PR removes the native mobile testsuite as whole.

Some parallel approaches to this one in these PRs:

• Remove gutenberg-mobile submodule while issues are sorted out #11301
• [WIP] Remove gutenberg-mobile submodule but keep CI tests #11304

How has this been tested?

• npm install, npm test work fine without pulling in any native mobile code
• Travis on this PR is green

Types of changes

• Removes the git submodule pointint to the https://github.com/wordpress-mobile/gutenberg-mobile
• Removes the 3 Travis jobs running the native mobile testsuite
• Removes the documentation related to running or debugging the native mobile tests

Checklist:

• My code is tested.
• My code follows the WordPress code style.
• My code follows the accessibility standards.
• My code has proper inline documentation.

[aduth]

Reverts #9883

[aduth]

LGTM 👍