[tests] remove 'COMMENT: no diff' from gmp test cases

src/plugins/e-acsl/tests/gmp/arith.i

src/plugins/e-acsl/tests/gmp/array.i

src/plugins/e-acsl/tests/gmp/at.i

src/plugins/e-acsl/tests/gmp/cast.i

src/plugins/e-acsl/tests/gmp/comparison.i

src/plugins/e-acsl/tests/gmp/integer_constant.i

src/plugins/e-acsl/tests/gmp/longlong.i

src/plugins/e-acsl/tests/gmp/not.i

src/plugins/e-acsl/tests/gmp/oracle/arith.0.res.oracle

src/plugins/e-acsl/tests/gmp/oracle/arith.1.res.oracle

src/plugins/e-acsl/tests/gmp/oracle/array.0.res.oracle

src/plugins/e-acsl/tests/gmp/oracle/array.1.res.oracle

src/plugins/e-acsl/tests/gmp/oracle/at.0.res.oracle

src/plugins/e-acsl/tests/gmp/oracle/at.1.res.oracle

src/plugins/e-acsl/tests/gmp/oracle/gen_arith.c

src/plugins/e-acsl/tests/gmp/oracle/gen_arith2.c

src/plugins/e-acsl/tests/gmp/oracle/gen_array.c

src/plugins/e-acsl/tests/gmp/oracle/gen_array2.c

src/plugins/e-acsl/tests/gmp/oracle/gen_at.c

src/plugins/e-acsl/tests/gmp/oracle/gen_at2.c