Remove needless, wrongly-placed system_test.permissions.yml file

In #2513626: [Regression] Module permission links missing from module list page, we discovered a system_test.permissions.yml file in core/modules/system/tests/modules/system_test/src that was added in #2464659: Routes that are varied by the 'user.permissions' cache context for anonymous users must also get the anonymous Role's cache tag. Permissions YAML files don't belong in "src" directories -- the permissions.yml file should be located in the module's root directory. The tests relying on those permissions still pass because the testing process doesn't actually require it.

If it is removed and tests pass, it is not needed.