perf tools: Unify a bit the build directory output

perf tools: Unify a bit the build directory output

Removing the extra 'SUBDIR' line from clean and doc build output.
Because it's annoying.. ;-)

Before:

  $ make clean
  ...
  SUBDIR   Documentation
  CLEAN    Documentation

After:

  $ make clean
  ...
  CLEAN    Documentation

Before:

  $ make doc
  BUILD:   Doing 'make -j8' parallel build
  SUBDIR   Documentation
  ASCIIDOC perf-stat.html
  ...

After:

  $ make doc
  BUILD:   Doing 'make -j8' parallel build
  ASCIIDOC perf-stat.html
  ...

Signed-off-by: Jiri Olsa <jolsa@kernel.org>
Tested-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Michael Petlan <mpetlan@redhat.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: http://lore.kernel.org/lkml/20200318204522.1200981-1-jolsa@kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>