Remove ESLint rule for OSS to X-Pack restricted paths

This is no longer necessary now that we validate OSS to X-Pack plugin
dependencies in the plugins service. Since there is no way to
differentiate "optional" and "required" dependencies in ESLint rules,
this rule needs to be removed to allow such optional dependencies.

.eslintrc.js

@@ -470,11 +470,6 @@ module.exports = {
                 errorMessage:
                   'Server modules cannot be imported into client modules or shared modules.',
               },
-              {
-                target: ['src/**/*'],
-                from: ['x-pack/**/*'],
-                errorMessage: 'OSS cannot import x-pack files.',
-              },
               {
                 target: ['src/core/**/*'],
                 from: ['plugins/**/*', 'src/plugins/**/*'],