perf tools: Remove check for unused PERF_PAGER_IN_USE

PERF_PAGER_IN_USE doesn't seem to be used anywhere, so let's remove it.

This will also make it easier to move pager.c into a separate library.

Signed-off-by: Josh Poimboeuf <jpoimboe@redhat.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: http://lkml.kernel.org/r/ed9e8370db9811746dc590544cf48c36dcfb1731.1449965119.git.jpoimboe@redhat.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>