Remove unused ImportCppId and list of Cpp imports in File (#6290)

See discussion:
https://discord.com/channels/655572317891461132/655578254970716160/1432518191350808659

Part of #5245.

toolchain/sem_ir/file.cpp

@@ -51,7 +51,6 @@ File::File(const Parse::Tree* parse_tree, CheckIRId check_ir_id,
       // The `2` prevents adding a tag for the global ids
       // `ImportIRId::{ApiForImpl,Cpp}`.
       import_irs_(IdTag(check_ir_id.index, 2)),
-      import_cpps_(check_ir_id),
       clang_decls_(check_ir_id),
       // The `+1` prevents adding a tag to the global `NameSpace::PackageInstId`
       // instruction. It's not a "singleton" instruction, but it's a unique