Changes

Remove unused private API log_target_create_file_stream()

This function was never added to the logging.h (despite it was not
static and it was inside libosmocore.map).
It is actually used nowhere, and changing it to static made the compiler
warn about it. Remove it.

Change-Id: Ia30c6b47cfcbb54e7c2c43b877cd9554dc596abf