Remove IE11 hack from fieldset.css

Problem/Motivation

The following CSS hack applies to IE11 only:

_:-ms-fullscreen,
.fieldset {
  display: table;
  box-sizing: border-box;
  width: 100%;
}

We no longer support IE11 so we can remove this rule.

Steps to reproduce

Proposed resolution

Remaining tasks

User interface changes

API changes

Data model changes

Release notes snippet