Simplify filepath init code

It will no longer attempt to use /home/janssege/.gnucash. That was
requiring lots of extra conditions.
It will also default to a base directory (gnc_data_home) in the
build dir if it detects the code is run during building or testing.
That again allows to simplify it as there's no need for temp dir
juggling in case the build environment doesn't have a writable home dir.

libgnucash/core-utils/gnc-filepath-utils.h

@@ -87,22 +87,13 @@ gchar *gnc_path_find_localized_html_file (const gchar *file_name);
  * This function should be called very early at startup (before any
  * other of the user data directory related function gets called).
  *
- * @param create If true the function will attempt to create the
- * gnucash user data directory its subdirectories and parent directories
- * if they don't exist yet. Note it won't attempt to create a home directory
- * if that is missing. In that case the system's default temporary
- * directory will be used instead. If false it will not attempt to create
- * any directories at all unless they are in the temporary directory. This
- * is a fallback measure to allow the calling application to more or less
- * function even if gnc_filepath_init was never called.
- *
  * @note If the necessary directories did get created this
  * function will also try to copy files from $HOME/.gnucash
  * to there if that old location exists.
  *
  * @return whether files got copied from the old location.
  */
-gboolean gnc_filepath_init(gboolean create);
+gboolean gnc_filepath_init (void);
 
 const gchar *gnc_userdata_dir (void);
 gchar *gnc_build_userdata_path (const gchar *filename);