Various buffers that produce compilation results or run tests on code apply an underline to the file names they reference or to relevant messages. Users may consider this unnecessary or excessive.
To outright disable the effect, use this (buffers need to be generated anew):
(setq compilation-message-face nil)
If some element of differentiation is still desired, a good option is to
render the affected text with the italic face:
(setq compilation-message-face 'italic)