remove reserved 'api' location

this is unused and we have no plans to use it.

src/constants.js

@@ -1,7 +1,6 @@
 'use strict';
 
 exports = module.exports = {
-    API_LOCATION: 'api', // this is unused but reserved for future use (#403)
     SMTP_LOCATION: 'smtp',
     IMAP_LOCATION: 'imap',
 

src/domains.js

@@ -145,7 +145,6 @@ function validateHostname(location, domainObject) {
     const hostname = fqdn(location, domainObject);
 
     const RESERVED_LOCATIONS = [
-        constants.API_LOCATION,
         constants.SMTP_LOCATION,
         constants.IMAP_LOCATION
     ];

src/routes/test/apps-test.js

@@ -381,13 +381,13 @@ describe('App API', function () {
             });
     });
 
-    it('app install fails - reserved api location', function (done) {
+    it('app install fails - reserved smtp location', function (done) {
         superagent.post(SERVER_URL + '/api/v1/apps/install')
             .query({ access_token: token })
-            .send({ manifest: APP_MANIFEST, location: constants.API_LOCATION, accessRestriction: null, domain: DOMAIN_0.domain })
+            .send({ manifest: APP_MANIFEST, location: constants.SMTP_LOCATION, accessRestriction: null, domain: DOMAIN_0.domain })
             .end(function (err, res) {
                 expect(res.statusCode).to.equal(400);
-                expect(res.body.message).to.contain(constants.API_LOCATION + ' is reserved');
+                expect(res.body.message).to.contain(constants.SMTP_LOCATION + ' is reserved');
                 done();
             });
     });