Drop gtk-doc dependency
    
    gtk-doc is not used anymore in mutter: gi-docgen is now used for
    building the documentation.
    Not replacing the dependency because API doc generation has been dropped
    in commit 7b5a0bf5812dcb8b3d821d463b1f8ba353799c0d so gi-docgen is not
    going to be used during the build anyway.
    
    Signed-off-by: Sandro Bonazzola <sbonazzo@redhat.com>