tp: delete the legacy PerfettoSqlPreprocessor

With trace_processor routing through the syntaqlite-driven macro path
(see the previous CL), the handwritten preprocessor and the
LegacyImpl/constexpr/optional<> machinery that gated its existence
have no remaining production callers.

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

Android.bp/BUILD regen happens at upload time via tools/gen_all per
project convention.