Have any of the reputable scholars issued a fatwa for the demolition of the Prophet's grave (peace be upon him)?

None of the scholars have issued a fatwa to demolish the grave of the Prophet (peace be upon him). It has been reported that Sheikh Ibn Uthaymeen said that the dome built over the grave is wrong and no one approves of it, but this does not mean it should be demolished.

A distinction must be made between the Prophetic Chamber and the dome, which was built later. Visiting the Prophetic Chamber is recommended without any disagreement.

Sheikh Muhammad ibn Abd al-Wahhab was falsely accused of saying that the Prophet's chamber should be demolished, but he is innocent of that.

As for the inclusion of Aisha's chamber into the Mosque during the reign of Al-Walid ibn Abd al-Malik, some scholars disliked it. This was done out of necessity to expand the Mosque, and it occurred after the death of most of the Companions.