[ML] Classification results view: confusion matrix label column should not be colored

[alvarezmelissa87]

Found in #58596 (comment)

Kibana version:
latest master

Describe the bug:
Label column in the confusion matrix is sometimes colored like the rest of the table when it shouldn't be.

Steps to reproduce:
View classification job results confusion matrix for classification job where dependent_variable is a numerical value.

Expected behavior:
First/label column in confusion matrix should never be colored

Screenshots (if relevant):

[elasticmachine]

Pinging @elastic/ml-ui (:ml)