Remove headers not needed anymore due to recent removal of unused fun…

Remove functions that are not used anymore.

Looks like previously used helper functions, but not in use
anymore.

Found by clang-tidy [clang-diagnostic-unused-function] check,
and also are warned about while compiling.

#xls-build-gardener

PiperOrigin-RevId: 684214961

Fix dangling help message.

An ephemeral string in a flag help message ends up being assigned
to a string_view which holds the reference to that temporary.

Instead, keep the generated help message backed by storage, so that
it can be safely passed to a string_view.

#xls-build-gardener

PiperOrigin-RevId: 683838862

Fix typo in butterfly topology documentation.

Change "-tree" to "-fly" when needed and change "-" to "\-" to correctly render the italics.

PiperOrigin-RevId: 683485795

Make codegen implement shra as a function.

This allows us to bind the intermediate result `$signed(to_shift) >>> amount` to a reg with a defined width, which avoids the `$signed()` having a self-determined width, which is a lint finding.

Fixes google/xls#1642.

PiperOrigin-RevId: 682526771

[xls] Ensure sitofp and friends work on tensors too

Accidental omission from previous patch.

PiperOrigin-RevId: 682190931

Added minor additions to DSLX types, funcs, and ast nodes.

  - Additional casts for Types.
  - GetParamByName for Functions.
  - Overrideable default for AstNodeVisitorWithDefault.

PiperOrigin-RevId: 681662121

[xlscc] Don't const-prop so much during short circuiting.

PiperOrigin-RevId: 681159199

Make FifoConfig constexpr-constructible.

PiperOrigin-RevId: 679331734

Make the default dslx stdlib path an `inline constexpr std::string_vi…