fixRegression: Fix #21586. (1). Remove some verbose and unnecessary code. (2). Remove the breaking change to `visualMap[type='continuous'].formatter: function () {}` - not necessary, and that breaking change can cause the existing lables display incorrectly when labels touch. (3). Switch to a more comprehensive implementation - adjust the positions of the two labels to avoid overlap. The previous approach (merging two labels) may occupy extra horizontal space and may unexpectedly cause overlap with other content. (4). Add visual test for `visualMap[type='continuous']`