[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