general: Remove obsolete SVN references.

There are a handful of files in the tree that
reference an SVN link for the coding guidelines.

This removes these because the links are dead
and the vast majority of source files do not
contain these links, so this is more consistent.

app_skel still maintains an (up to date) link
to the coding guidelines.

ASTERISK-30159 #close

Change-Id: I35bbb20f66982e98099cff3029ede20091ffdac7