Getting rid of 'oldref' in the help files

Hi Cloph, all,

I've recently proposed some help cleanups on the documentation@ ML, and
this is the first one of them.  I'm cross-posting to l10n@ and
documentation@ - this change is supposed to be transparent for L10n and
Documentation teams, but they should know :-)

The idea is to get rid of the 'oldref' attribute in the help files; ie.
change

  <paragraph role="heading" id="hd_id3145649" xml-lang="en-US" level="2" l10n="U" 
oldref="13">Heading</paragraph>

to

  <paragraph role="heading" id="hd_id3145649" xml-lang="en-US" level="2" 
l10n="U">Heading</paragraph>

The 'oldref' comes from helpcontent -> helpcontent2 migration, and the
documentation says "This contains the reference number used by the old
help files and is only used for migration purposes."

Unfortunately, it is used in the msgctx flag in the .po files, like:

#: main0503.xhp
msgctxt ""
"main0503.xhp\n"
"hd_id3155084\n"
"21\n"
"help.text"
msgid "Flexible Application Interface"
msgstr "Snadno přizpůsobitelné uživatelské rozhraní"

The "21\n" above is the oldref.

As we talked on the IRC - unless there are any objections, can you
please do your magic with the next translation update so that we remove
these oldrefs from the helpcontent, the .po templates, and .po
translations themselves?

The helpcontent2 part of that is this:

  git grep -l 'oldref="[0-9]*"' | xargs sed -i 's/ *\<oldref="[0-9]*" *//'

Thank you in advance!

[Or - any objections to this change?]

All the best,
Kendy