Convert gl-*-inherit mixins to gl-reset-* mixins

In !1319 (merged), we have introduced a naming pattern for mixins that reset specific properties:

@mixin gl-reset-text-align {
  text-align: inherit;
}

In other places we have similar mixins that also reset properties to the inherit value, but those use a different naming convention:

@mixin gl-color-inherit {
  color: inherit;
}

@mixin gl-text-inherit {
  color: inherit;
}

@mixin gl-text-decoration-color-inherit {
  text-decoration-color: inherit;
}

Let's:

• Rename those mixins to use the new naming pattern.
• Deduplicate gl-color-inherit and gl-text-inherit that both do the same thing.

We should end up with the following mixins:

@mixin gl-reset-color {
  color: inherit;
}

@mixin gl-reset-text-decoration-color {
  text-decoration-color: inherit;
}

/cc @dpisek @ealcantara @sarahghp