Remove special GTest handling for ROS

[sloretz]

Description

We're deprecating the gtest_vendor and gmock_vendor packages in ROS. You can now use the libgtest-dev rosdep key

ament/googletest#37

I'd like this backported to 2.3.x for ROS 2 https://github.com/ros2/ros2/blob/29edd6c3ea3510ce77f5e41febe1148eaddf4b85/ros2.repos#L30-L33

Contributor Checklist

• Commit messages follow the project guidelines.
• The code follows the style guidelines of this project.
• Tests that thoroughly check the new feature have been added/Regression tests checking the bug and its fix have been added; the added tests pass locally
• Any new/modified methods have been properly documented using Doxygen.
• Changes are backport compatible: they do NOT break ABI nor change library core behavior.
• Changes are API compatible.
• New feature has been added to the versions.md file (if applicable).
• Applicable backports have been included in the description.

Reviewer Checklist

• The PR has a milestone assigned.
• The title and description correctly express the PR's purpose.
• Check contributor checklist is correct.
• Check CI results: changes do not issue any warning.
• Check CI results: CI pass and failing tests are unrelated with the changes.

[MiguelCompany]

LGTM with green CI

[MiguelCompany]

@sloretz Thank you.

I'd like this backported to 2.3.x for ROS 2

2.3.x is currently a mirror of master, so this is automagically done.