Remove BinaryOperatorAdd (#3469)

BinaryOperatorAdd had been added early on as a proof-of-concept for
toolchain design for parsing -> checking flow. However, it doesn't
reflect the interface direction for operators, and now other portions of
the toolchain demonstrate the relevant logic. Instead, it's just a bit
of an outlier versus other instructions which have evolved over time.

toolchain/check/context.cpp

@@ -830,7 +830,6 @@ class TypeCompleter {
       case SemIR::ArrayInit::Kind:
       case SemIR::Assign::Kind:
       case SemIR::BaseDecl::Kind:
-      case SemIR::BinaryOperatorAdd::Kind:
       case SemIR::BindName::Kind:
       case SemIR::BindValue::Kind:
       case SemIR::BlockArg::Kind: