tools/perf/build: Remove the volatile-register-var feature check

tools/perf/build: Remove the volatile-register-var feature check

Namhyung Kim noticed that the volatile-register-var feature check
is superfluous:

 > The gcc manpage says this warning is enabled by -Wall, and we add -Wall
 > to CFLAGS before doing feature checks.  So all gcc versions that support
 > -Wvolatile-register-var enables it by default without this check and
 > older gcc versions will always fail the feature check.

Remove it - this will further speed up feature checks.

Reported-by: Namhyung Kim <[email protected]>
Cc: Arnaldo Carvalho de Melo <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Cc: David Ahern <[email protected]>
Cc: Jiri Olsa <[email protected]>
Signed-off-by: Ingo Molnar <[email protected]>
Signed-off-by: Arnaldo Carvalho de Melo <[email protected]>