[clang] remove (clang::)ast_matchers:: namespace from AST matcher args for docs (#81437)

When parsing the ASTMatchers.h file, a matcher could specify an argument
that is a matcher using the not needed namespace
`(clang::)ast_matchers::`.
Change the argument parsing in dump_ast_matchers.py to remove those
namespaces such that when parameters with these namespaces slip through,
the namespaces will be not be shown in the matchers reference, like it
is done with the `internal` namespace.
Additionally, remove the not needed namespaces from arguments in
ASTMatchers.h.