gh-116946: remove unnecessary gc from _decimal types

[skirpichev]

On my laptop (gcc 14.2, default configure flags) for pyperformance benchmarks (decimal_* should be enabled first, see python/pyperformance#473):

decimal_factorial: Mean +- std dev: [ref] 1.05 sec +- 0.02 sec -> [patch] 918 ms +- 6 ms: 1.14x faster
decimal_pi: Mean +- std dev: [ref] 1.99 sec +- 0.02 sec -> [patch] 1.76 sec +- 0.02 sec: 1.13x faster
telco: Mean +- std dev: [ref] 78.7 ms +- 0.5 ms -> [patch] 73.5 ms +- 0.4 ms: 1.07x faster

Geometric mean: 1.11x faster

• Issue: Heap types implemented in C must have Py_TPFLAGS_HAVE_GC and implement tp_traverse #116946

[skirpichev]

This partially fix speed regression, noticed in #144650.