B2STauTau 3pi3pi lines (bugfix naming conflict)

Addressing conflict mentioned by @yitang at !1832 (comment 6938311).

The selection with the name Combine_Tau3pi_DDK existed in both StrippingB2XTauTau.py and StrippingB2STauTauInclusive.py. A commit in the B2TauNu MR fixed this, but this was overwritten by the B2STauTau MR which for some reason didn't have this fix.

Thanks to @yitang and @yusong for catching this.