Fix issue #6669 - Remove the css implementation from the global HTML button element

[Nemila]

1. The styling of the button should be in _buttons.scss not in _forms.scss .
2. _button.scss already have a :focus state.
3. The styling shouldn't be applied to the global HTML button element and should instead be applied to the .btn class.

Proposed changes

Screenshots (if appropriate) or codepen:

Types of changes

• Bug fix (non-breaking change which fixes an issue).
• New feature (non-breaking change which adds functionality).
• Breaking change (fix or feature that would cause existing functionality to change).

Checklist:

• I have read the CONTRIBUTING document.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have added tests to cover my changes.
• All new and existing tests passed.