[LLDB][Clang Integration][NFC] Remove redundant condition

Condition omit_empty_base_classes is checked both in an outer and
in an inner if statement in TypeSystemClang::GetNumBaseClasses().
This patch removes the redundant inner check.

The issue was found using clang-tidy check under review
misc-redundant-condition. See https://reviews.llvm.org/D81272.

Differential Revision: https://reviews.llvm.org/D82559