Remove notice about civilint from `testing/index.md`

Running civilint is important for any/all code.  Seems a bit arbitrary
emphasizing this point specifically for tests.

docs/testing/index.md

 The easiest way to get set up to run the automated tests is to use [buildkit](/tools/buildkit.md) which will install the tools necessary for running the tests and creating local CiviCRM sites with [civibuild](/tools/civibuild.md).
-
-!!! info
-    When writing new tests or making any changes make sure you run [civilint](/tools/civilint.md)
-    to ensure your changes match our coding style.