What technique is used in this quote from Act 5, Scene 5: "The time has been my senses would have cooled / to hear a night shriek"?

After Seyton has left the stage to investigate a strange scream, Macbeth speaks the following lines,

I have almost forgot the taste of fears.

The time has been my senses would have cooled

To hear a night-shriek, and my fell of hair 

Would at a dismal treatise rouse and stir

As life were in 't. (5.5.11-15)

In other words, he says that he almost forgot what fear feels like. There was once a time when...

After Seyton has left the stage to investigate a strange scream, Macbeth speaks the following lines,

I have almost forgot the taste of fears.

The time has been my senses would have cooled

To hear a night-shriek, and my fell of hair 

Would at a dismal treatise rouse and stir

As life were in 't. (5.5.11-15)

In other words, he says that he almost forgot what fear feels like. There was once a time when he would have been very frightened to hear a cry in the night, and his hair would have stood up by itself, as though it were alive, to hear something scary. However, such a scream no longer affects him this way.

Because Macbeth is alone on stage when he speaks these lines, they are considered part of a soliloquy. A soliloquy is when a character speaks alone on stage, and because they are alone, this is an opportunity for the audience to find out what that character is truly thinking and feeling; it's like thinking out loud. It seems likely that Macbeth would say what he does when he's alone because only he realizes how much he has changed since the beginning of the play. He has experienced so many horrors -- horrors that he has committed or commissioned -- that he no longer quails at things that would have frightened him before. He would not admit this to another person.

Also, to say that "my senses would have cooled" is figurative language because fear doesn't actually make us cold, although it can make us feel cold. For this reason, I would also suggest that the line is an example of metonymy, a figure of speech in which a detail associated with something is substituted for the thing with which it is associated.  Macbeth is substituting the sensation of being made cold to convey the literal meaning that he would have felt afraid.