[Demangle] remove itaniumFindTypesInMangledName

Summary:
This (very specialized) function was added to enable an LLDB use case.
Now that a more generic interface (overriding of parser functions -
D52992)  is available, and LLDB has been converted to use that (D54074),
the function is unused and can be removed.

Reviewers: erik.pilkington, sgraenitz, rsmith

Subscribers: mgorny, hiraditya, christof, libcxx-commits, llvm-commits

Differential Revision: https://reviews.llvm.org/D54893

llvm-svn: 347670