Revert "coverage - ignore file" addition because it results in less flexibility

[JanMichaelPeter]

Describe the bug
We added a line "coverage - ignore file" to generated code in #104
This works fine if widget driver is the only generator that produces code. But for example if I combine widget_driver and json_serializable I run into a problem:
widget_driver writes the ignore coverage comment by default while I need to customize the build_runner from json_serializable with a preamble inside build.yaml see this reference. If I do both, I end up with two "coverage - ignore file" comments in all widget_driver generated files.

To Reproduce
Steps to reproduce the behavior, e.g.:

1. have json_serializable and widget_driver_generator in class
2. create a driver that triggers the generation of a file
3. add this to the build.yaml

targets:
  $default:
    builders:
      source_gen:combining_builder:
        options:
          preamble: |
            // coverage:ignore-file

4. run dart run build_runner build --delete-conflicting-outputs
5. Observe two "coverage - ignore file" comments in generated driver files
   Expected behavior
   No "coverage - ignore file" comments in generated driver on default. I want only the option to set it with the pramble