fix: platform remove command fails with old Android runtime

[rosen-vladimirov]

After upgrading nativescript-doctor, the version of the Java is
validated against the current Android runtime version. In case the
runtime version cannot work with Java 10, nativescript-doctor does not
allow the operation to continue. This breaks the platform remove
command, as it checks the system requirements first, before executing
the actual removal. However, this is no longer required as the code for
removing the platform is try-catched and in case the current
environment is not setup correctly, it will try to remove the
platforms/ dir and the respective key in package.json.
So remove the environment validation from this command. Also remove the
validatePlatformInstalled call from the canExecute method of the
command, as it is actually called in the removePlatforms method.

PR Checklist

• The PR title follows our guidelines: https://github.com/NativeScript/NativeScript/blob/master/CONTRIBUTING.md#commit-messages.
• [] There is an issue for the bug/feature this PR is for. To avoid wasting your time, it's best to open a suggestion issue first and wait for approval before working on it.
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• All existing tests are passing: https://github.com/NativeScript/nativescript-cli/blob/master/CONTRIBUTING.md#contribute-to-the-code-base
• Tests for the changes are included.

What is the current behavior?

Trying to execute tns platform remove android on an older project, when Java version on local machine is 10, fails with error that you need to update your Android runtime version.

What is the new behavior?

You can successfully remove older platforms with tns platform remove android even when Java version is 10.