Is it illegal to remove them once you purchase the pistol?

The law only affects sales, rentals and transfers by dealers:

Maryland Code  § 5-132 - Handgun safety devices: (c) (2) On or after January 1, 2003, a dealer may not sell, offer for sale, rent, or transfer in the State a handgun manufactured on or after January 1, 2003, unless the handgun has an integrated mechanical safety device.

There is nothing prohibiting you, as a private owner, from removing or permanently disabling the integrated safety device on your handgun.  I also don't read any prohibition against having a gunsmith do it for you since no sale, rental or transfer is involved.

The only meaningful consequence that I do see is that you would be unable to sell the handgun within Maryland once you remove the integrated safety device.  Handguns, being regulated weapons, must be transferred through a dealer when sold.  This even applies to private sales.  Thus the integrated safety device law would generally apply to any sale of the handgun within Maryland.  There are some exceptions, but you should count on not being able to sell it in Maryland.

I suppose there is also a possibility that if someone were accidentally injured by your modified handgun, a lawyer in a civil case might argue that removal of the device was negligent on your part.