Remove specific height of .form-select

Created by: ysds

Those heights have been setted for fix the #18842 (closed). But it should not be needed for .form-selects.

I'll check in BrowserStack.

demo:

• https://codepen.io/anon/pen/NQdVdX
• https://deploy-preview-29167--twbs-bootstrap.netlify.com/docs/4.3/forms/select/
• https://deploy-preview-29167--twbs-bootstrap.netlify.com/docs/4.3/forms/input-group/