Make GCC brief format the only output format during parsing.

Parser error messages are the ones emited from lex/yacc. Historically,
they have been emitted in a special CFEngine format. This led to the
introduction of the --gcc-brief-format flag to output errors in a way
that most tools/editors can pick up.

This change removes the historical CFEngine output format, makes brief
format the only output format, and removes the --gcc-brief-format flag.