Replace comment preservation with simple comment stripping for format tests

[kyleconroy]

Instead of adding complex comment handling to the AST (Comment struct,
StatementWithComments wrapper), use a simple comment-stripping function
in the test to normalize expected output. This approach:

• Removes 57 lines from ast/ast.go (Comment, StatementWithComments)
• Simplifies the parser by just skipping comments instead of tracking them
• Removes comment formatting code from the format package
• Adds a simple stripComments() function in tests for expected output normalization

The format tests now compare the formatted output against the original
query with comments stripped, which is simpler and achieves the same goal.