[TableGen][GISel] Delete unused `Src` arguments (NFC) (#120445)
The last uses were removed in #120332 and #120426.
When emitting renderers, we shouldn't look at the source DAG at all. The
required information is provided by the destination DAG and by the
instructions referenced in that DAG. Sometimes, we do want to know if a
result was referenced in the source DAG; this can be checked by calling
`RuleMatcher::hasOperand`. Any other use of the source DAG when emitting
renderers is likely an error.
Pull Request: https://github.com/llvm/llvm-project/pull/120445