tp: delete the legacy PerfettoSqlPreprocessor (#5472)

**Stack:**

1. #5614 — `tp: prep PerfettoSqlParser for a second backend`
2. #5615 — `tp: add syntaqlite-driven macro expansion behind a flag`
3. #5525 — `tp: replace sqlfluff formatter with syntaqlite` (parallel;
based on #5615)
4. #5616 — `tp: route PerfettoSqlParser through syntaqlite macro
expansion`
5. **#5472 — `tp: delete the legacy PerfettoSqlPreprocessor`** ← you are
here

---

Final PR in a 4-PR stack that splits the original "replace the
handrolled preprocessor with syntaqlite" change. With trace_processor
routing through the syntaqlite-driven macro path (see the previous PR in
the stack), the handwritten preprocessor and the
`LegacyImpl`/`constexpr`/`std::optional<>` machinery that gated its
existence have no remaining production callers.

This PR:

 * Deletes `src/trace_processor/perfetto_sql/preprocessor/` entirely
   (~2.6k lines of generated grammar + handwritten preprocessor +
   tests) and drops its `BUILD.gn` / `Android.bp` / `BUILD` references
   from the parser, engine, and root `trace_processor` targets.
 * Inlines the real `Macro` struct on `PerfettoSqlParser` (was a
   `using` alias to the preprocessor type) and drops the parser/engine
   includes of the preprocessor header.
 * Collapses `PerfettoSqlParser::Impl` from the dual-impl wrapper into
   the syntaqlite implementation directly: no more `LegacyImpl`,
   `kUseSyntaqliteMacros` / `kUsesSyntaqliteMacros`, `std::optional<>`
   slots, or `if constexpr` dispatch.
 * Drops the `kBang` token alias added to `SqliteTokenizer` when the
   legacy preprocessor needed to recognise `!` as a macro indicator.
 * Drops the `kUsesSyntaqliteMacros` test gate and the
   `ExpandsTokenApplyIntrinsic` skip-on-legacy guard.
 * Refreshes the engine.cc architecture comment to drop the
   preprocessor layer from the diagram.

The earlier PRs in the stack:

 1. `tp: prep PerfettoSqlParser for a second backend` — pure refactor.
 2. `tp: add syntaqlite-driven macro expansion behind a flag` — adds
    `SyntaqliteMacroImpl` + `intrinsic_macro_expansion.{h,cc}` +
    `SqlSource::FromMacroExpansion` + the syntaqlite library bump,
    behind `kUsesSyntaqliteMacros = false`.
 3. `tp: route PerfettoSqlParser through syntaqlite macro expansion` —
    flips the flag.