DomTree: Remove the releaseMemory() method

Summary:
It is fully redundant with reset().

Change-Id: I25850b9f08eace757cf03cbb8780e970aca7f51a

Reviewers: arsenm, RKSimon, mehdi_amini, courbet

Subscribers: wdng, cfe-commits, llvm-commits

Tags: #clang, #llvm

Differential Revision: https://reviews.llvm.org/D83084