aria-label should not be set automatically for xc-button

[PatrickVonMassow-GIP]

Currently, the component xc-button automatically sets the aria-label as the value of the tag's content. There are two problems with this:

• An aria-label should only be set when needed. In this case, it's unnecessary, because a screen reader would read the tag's content when no aria-label is set. A case where an aria-label is necessary is an xc-icon-button because it has no text.
• In case of the xc-button, setting the aria-label is not only bad practice, but the way it's currently done is incorrect, because the aria-label is being set to the language key of the tag's content, instead of the translated value.

Solution: For xc-button, don't automatically set the aria-label to the tag's content. This still preserves the option to manually set an aria-label that differs from the tag's content, because xc-button has an optional input attribute xc-button-aria-label for this purpose.