In a Tale of Two Cities, why do Miss Pross and Jarvis Lorry destroy Dr. Manette's shoemaking tools, in the second book?

On the morning of Lucie and Charles's wedding in Chapter
XVIII of Book the Second of A Tale of Two Cities, Darnay goes to
speak with Dr. Manette per their agreement in Chapter X.  Shortly thereafter, Darnay and
the Doctor emerge, but it is a pale, albeit composed Manette that Mr. Lorry
witnesses.

After the wedding, the Doctor, Mr. Lorry, and
Miss Pross are left alone.  Mr. Lorry observes the old frightened and absent look upon
his friend's visage; Manette's way of grasping his head and wandering away recalls to
Mr. Lorry his first sight of the Doctor at the wine-shop. Shortly after Manette retires
to his room, Miss Pross and Mr. Lorry hear the pounding of his hammer.  So, Mr. Lorry
ascends to the room and calls to Manette, who does not recognize him; instead, he
continues to fashion the lady's shoe on which he works.  Because of Manette's reaction,
Mr. Lorry determines that the Doctor requires rest.  He and Miss Pross devise a plan to
ensure that he get this rest by writing to Lucie, who is on her honeymoon, that he has
been called away professionally, and by telling others that the Doctor is not
well.

After nine days of Manette's shoemaking during which
Mr. Lorry has closely observed him and tried to draw him away from his tasks, Dr.
Manette continues at his shoemaking in an even more skillful manner. However, on the
next day, Mr. Lorry, who has fallen asleep during his vigil, awakens to find that
Manette has put aside his tools and is reading by the window.  At first, Mr. Lorry
begins to doubt himself, wondering if he has merely imagined that Dr. Manette was thus
engaged?  But, then, after conferring with Miss Pross, Mr. Lorry is convinced of the
reality of Manette's condition.  So, again Mr. Lorry speaks to Manette in his
business-like third person, explaining that there has been a relapse in the man about
whom he has earlier conferred with the doctor.  Dr. Manette
confides,

"I
believe...that there had been a strong and extraordinary revival of the train of thought
and remembrance that was the first cause of the malady.  Some intense associations of a
most distressing nature were vividly recalled, I
think."

Further, he tells Mr.
Lorry that he does not believe that anything other that the "one train of association"
would renew the illness.  But, when Mr. Lorry suggests that the tools be removed,
Manette explains that there is yet a sense of terror that the tools, which once were the
only comfort to this man during his imprisonment, be removed.  Persistent, Mr. Lorry
suggests that these tools are a reminder of the man's fear.  "I would not keep it." 
Reluctantly, Dr. Manette agrees,

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"In her name, then, let it e done; I sanction
it.  But, I would not take it away while he was present....let him miss his old
companion after an
absence."

Therefore, he and
Miss Pross determine on the fourteenth day while the Doctor visits Lucie and her
husband that it is best to destroy the shoemaking tools, the reminder of Manette's
imprisonment and torture.