Reboot for focused placeholder link

[ysds]

The focus states are taken into consideration, but I have a doubt about the necessity of them.

bootstrap/scss/_reboot.scss

Lines 199 to 211 in 39b7686

	a:not([href]):not([tabindex]) {
	color: inherit;
	text-decoration: none;
	@include hover-focus {
	color: inherit;
	text-decoration: none;
	}
	&:focus {
	outline: 0;
	}
	}

It can be removed IMO. After:

a:not([href]) {
  color: inherit;
  text-decoration: none;

  @include hover {
    color: inherit;
    text-decoration: none;
  }
}

Related to: #21633, #21487, #19411

Any thoughts?

P.S: If we have plain-hover() mixin like the plain-hover-focus() mixin, can be more simplified:

a:not([href]) {
  @include plain-hover {
    color: inherit;
    text-decoration: none;
  }
}