[Modules] Remove unnecessary check when generating name lookup table in ASTWriter

[Modules] Remove unnecessary check when generating name lookup table in ASTWriter

Close https://github.com/llvm/llvm-project/issues/61065.

We will avoid writing the names from external AST naturally. But
currently its check is often false positive since we may have already
marked the declarations as external but
DeclContext::hasNeedToReconcileExternalVisibleStorage would be false
after reconciling.

Tested with libcxx's modular build.

This patch can improve 8% compilation time in an internal workloads.