Code quality improvements driven by JETLS diagnostics

[aviatesk]

This PR bundles a series of small cleanups prompted by running JETLS over the codebase.
None of the exported API changes; all signature changes are to internal (non-exported) helpers.

• Remove unused variables and dead code (db0a6cc)
• Separate expr and Frame method-query overloads (8881ea6):
  ismethod(::Frame) / ismethod3(::Frame) are renamed to is_frame_at_method / is_frame_at_method3.
  They answer a different question from ismethod(stmt) (which inspects a statement), and sharing the name introduced unnecessary union splits due to callsite argument type refinements.
  Revise only calls ismethod on statements, so it is unaffected.
• Use === for symbol comparison (1f3895f)
• Add restrictive argument type declarations to internal subroutines (fe53c94)
  • Annotate isrequired::AbstractVector{Bool}, objs::Set{GlobalRef}, idx::Int, etc. on the
    lines_required! worker and its helpers (add_requests!, add_ssa_preds!, add_named_dependencies!,
    add_preds!, add_obj!, add_typedefs!). This improves JETLS diagnostics and documents intent.
  • Note: the 5-positional-arg lines_required!(isrequired, objs, src, edges, controller) now requires objs::Set{GlobalRef}. This overload is not covered by the docstring and every public entry point already passes a Set{GlobalRef}; only one test needed updating.
  • Add @nospecialize to ismethod/ismethod1/ismethod3 and type ismethod_with_name's src::CodeInfo.
• Minor cosmetic changes (d05aa78)
  • Guard the "no method found" @warn in methoddef! with loc !== nothing, so a missing line-table entry no longer raises instead of warning.
  • Rename unused arguments to _-prefixed names, tidy using order in tests.