Remove redundant "title" attribute from menu items

### Issue type

**I'm submitting a ...** (check one with "x")

* [ ] bug report
* [x] feature request

### Issue description

**Current behavior:**
We are using Nebular and evaluated our app with WAVE accessibility evaluation tool, and got warning "Redundant title text" for every item in the nb-menu. The root of the problem is that every menu item sets the `[attr.title]="menuItem.title"` title but the menu text is exactly the same. For further info: https://accessibilitytips.com/2008/04/14/avoiding-redundant-title-attributes/

**Expected behavior:**
Remove the title attribute.

### Other information:

this patch should fix the issue

[patch.txt](https://github.com/akveo/nebular/files/7309601/patch.txt)