removing @lsr{} and only using @lsrdir{}

[Graham Percival]

Does anybody really want to keep @lsr{} (in addition to
@lsrdir{})?  It isn't used in any finished GDP sections, and I'm
90% certain it isn't worth keeping.

@lsrdir{Pitches, Pitches}  points to the whole collection of pitch
snippets.
@lsr{pitches, adding-ambitus-per-voice.ly, Ambitus}  (or something
like that; I can't recall the exact syntax)  gives us a link
directly to that snippet.

I introduced @lsr{} back in the Bad Old Days when snippets were
sorted into directories; if you have a snippet that was useful for
two things, the only way to get it in two parts of the manual was
to use @lsr.  But nowadays, we can simply add two tags to the
snippet, and have it appear in both lists.

We could then rename @lsrdir{} to @rlsr{} and use the same format
as all the other @rfoo{} macros.  That would simply the doc
source.

Unless I hear voiciferous complaints before Monday, I'll remove
@lsr{}.

Cheers,
- Graham