[ThinLTO/WPD] Remove an overly-aggressive assert

Summary:
An assert added to the index-based WPD was trying to verify that we only
have multiple vtables for a given guid when they are all non-external
linkage. This is too conservative because we may have multiple external
vtable with the same guid when they are in comdat. Remove the assert,
as we don't have comdat information in the index, the linker should
issue an error in this case.

See discussion on D71040 for more information.

Reviewers: evgeny777, aganea

Subscribers: mehdi_amini, inglorion, hiraditya, steven_wu, dexonsmith, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D72648