Remove obsolete notes on master file grammar

[k0ekk0ek]

What it says on the tin. The text discusses how it's near impossible to use a parser framework intended for context-free grammers to write a zone file parser and justifies lack of performance by stating it does not matter for operation. While true, as NSD 4.10 will use simdzone, this comment is no longer relevant. As for the second statement, the parser is a big part of zone loading performance, so it most definitely matters. The database may be incredibly fast, if the data cannot be read fast, zone loading performance suffers. The same holds true for IXFRs, as those are stored in the presentation format as well.