Pectoralis muscles. Actually, this question should read "was", not "is", since we never perform radical mastectomies any more (nor have we for 30+ yrs); removing the muscles never helps treat the cancer. The most common mastectomy for breast cancer treatment is called a modified radical mastectomy. This means that the breast and armpit lymph nodes are removed without any muscle removal.
Pectoralis but... The pectoralis major muscle is removed with a radical mastectomy. A modified radical mastectomy (mrm) is the standard of care and does not remove the muscle, only the breast and the lymph nodes. These days removing the muscle is rarely done. I recommend getting a second opinion at a breast center for evaluation and recommendations.