perf build: Remove NO_LIBCAP that controls nothing

perf build: Remove NO_LIBCAP that controls nothing

Using libcap was removed in commit e25ebda78e230283 ("perf cap: Tidy up
and improve capability testing") and improve capability testing"),
however, some build documentation and a use of the NO_LIBCAP=1 were
lingering.

Remove these left over bits.

Fixes: e25ebda78e230283 ("perf cap: Tidy up and improve capability testing")
Signed-off-by: Ian Rogers <irogers@google.com>
Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Ian Rogers <irogers@google.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: James Clark <james.clark@linaro.org>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>