Revert "Fix crash from accessing a Check::Context during lowering (#7335)" (#7353)

This reverts commit afd679129d.

The commit introduced ASAN errors:

https://github.com/carbon-language/carbon-lang/actions/runs/27436787960/job/81100665405

toolchain/lower/context.cpp

@@ -5,7 +5,6 @@
 #include "toolchain/lower/context.h"
 
 #include "clang/Basic/SourceManager.h"
-#include "clang/Sema/MultiplexExternalSemaSource.h"
 #include "common/check.h"
 #include "common/growing_range.h"
 #include "common/raw_string_ostream.h"
@@ -13,7 +12,6 @@
 #include "llvm/Transforms/Utils/ModuleUtils.h"
 #include "toolchain/lower/file_context.h"
 #include "toolchain/sem_ir/inst_namer.h"
-#include "toolchain/sem_ir/read_only_ast_source.h"
 
 namespace Carbon::Lower {
 
@@ -72,23 +70,6 @@ auto Context::Finalize() && -> std::unique_ptr<llvm::Module> {
 
   for (auto& file_context : file_contexts_.values()) {
     if (file_context) {
-      if (file_context->cpp_file()) {
-        // Remove the `CarbonExternalASTSource` installed during check
-        // (always the last child of the multiplex source) and replace
-        // it with a `ReadOnlyASTSource`. This is necessary because the
-        // original source has a now-invalid pointer to a
-        // `Check::Context`.
-        auto& ast = const_cast<clang::ASTContext&>(
-            file_context->cpp_file()->ast_context());
-        auto* multiplex_source =
-            cast<clang::MultiplexExternalSemaSource>(ast.getExternalSource());
-        auto& child_sources = multiplex_source->GetSources();
-        child_sources.pop_back();
-        multiplex_source->AddSource(
-            llvm::makeIntrusiveRefCnt<SemIR::ReadOnlyASTSource>(
-                file_context->sem_ir()));
-      }
-
       file_context->Finalize();
     }
   }