Simply search for *egcc* (the stars are important) in the main FSX folder and you should see a list of files with EGCC in their name. Filenames beginning with AF2, AFX or BR2 are actual AFCAD files, the others will be scenery components so do not delete them. For any duplicates simply rename the ones you don't want by adding .off at the end of the filename i.e. AF2_EGCC.bgl should be renamed to AF2_EGCC.bgl.off, those files will now be disabled.
Oh yes, almost forgot, if you want to keep the MTX AFCADS then you want to keep the BR2 files, although if you have a third party add-on for Manchester from the likes of UK2000 for example (excellent product by the way) then you will want to keep their AF2 file instead rather than the BR2 one from Burkhard Renk.