perf doc: Remove references to user-manual

Perf doesn't have a user-manual.txt, but git does and this explains why
there are references here. Having these references breaks 'make info' as
user-manual.info can't be created given the missing dependency. Remove
all references to user-manual so that 'make info' can succeed.

Signed-off-by: Ian Rogers <irogers@google.com>
Acked-by: Namhyung Kim <namhyung@kernel.org>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Stephane Eranian <eranian@google.com>
Link: https //lore.kernel.org/r/20210715013343.2286699-4-irogers@google.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>