GCD_Poly_ML: deleted unused code for cancellation and add.of.fract

this code concerned a format of polynomials which turned out uneasy in 2003

src/Pure

src/FOL

src/HOL

src/ZF

src/CCL

src/CTT

src/Cube

src/FOLP

src/LCF

src/Sequents

src/Doc

src/Tools/isac