perf hist: Fix memory leak of a perf_hpp_fmt

perf hist: Fix memory leak of a perf_hpp_fmt

perf_hpp__column_unregister() removes an entry from a list but doesn't
free the memory causing a memory leak spotted by leak sanitizer.

Add the free while at the same time reducing the scope of the function
to static.

Signed-off-by: Ian Rogers <irogers@google.com>
Reviewed-by: Kajol Jain <kjain@linux.ibm.com>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Stephane Eranian <eranian@google.com>
Link: http://lore.kernel.org/lkml/20211118071247.2140392-1-irogers@google.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>