Fix: common: make lttng_trace_chunk_remove_subdirectory_recursive static

This function is only used in this file, make it static.  Fixes this
diagnostic:

  CC       trace-chunk.lo
/home/simark/src/lttng-tools/src/common/trace-chunk.c:1498:5: error: no previous prototype for ‘lttng_trace_chunk_remove_subdirectory_recursive’ [-Werror=missing-prototypes]
 1498 | int lttng_trace_chunk_remove_subdirectory_recursive(struct lttng_trace_chunk *chunk,
      |     ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Change-Id: I19e03b64557ce845b94f02fcf0d6fbafba654d95
Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>