Remove obsolete solc AST CLI flag

[nhatdo9825-cmyk]

Fixes #2501.

This removes the obsolete --solc-ast CLI flag, whose help text suggested that Slither could accept a JSON AST directly. Maintainer discussion in #2501 notes that this support was removed and should stay unavailable unless it is re-added properly in crytic-compile.

The PR also removes the CLI smoke test for that deprecated flag so the test suite no longer preserves the misleading option.

Validation:

• python -m py_compile slither_main_.py
• git diff --check
• rg solc_ast/--solc-ast/solc-ast across slither, scripts, tests, docs, README.md, pyproject.toml

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