tools build: Remove feature-libslang-include-subdir

tools build: Remove feature-libslang-include-subdir

Added in commit cbefd24f0aee3a5d ("tools build: Add test to check if
slang.h is in /usr/include/slang/") this feature was to fix build
support on now unsupported versions of RHEL 5 and 6. As 6 years has
passed let's remove the workaround.

Signed-off-by: Ian Rogers <irogers@google.com>
Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Ingo Molnar <mingo@redhat.com>
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>