Remove description from fieldsets in settings form

Problem/Motivation

The description in the current settings fieldset is useless at best, and there's even a typo in two of them that prevent them from being shown. When that typo is fixed, the result is worse as basically no useful information is provided, and the UI of the fieldset description places it at the bottom of all fields in the fieldset.

Steps to reproduce

See admin/config/people/tfa

Proposed resolution

Remove those descriptions.