Remove old Flex/Bison parser code

[renecannao]

The old Flex/Bison-based POC parser is fully superseded by the new hand-written parser. Remove:

• `src/mysql_parser/` — Flex lexer + Bison grammar + parser.cpp
• `src/pgsql_parser/` — same for PostgreSQL
• `include/mysql_parser/` — old AST and parser headers
• `include/pgsql_parser/` — same for PostgreSQL
• `examples/` — old example programs
• `Makefile` — old build system (rename `Makefile.new` to `Makefile`)

Should be done after the new parser has proven itself in testing.

[renecannao]

Old Flex/Bison code removed. Makefile.new renamed to Makefile. All references updated.