[IA64] perfmon: Remove exit_pfm_fs()

Because it is dead code and not referenced by anybody else (that file cannot
be built modular).

Signed-off-by: Satyam Sharma <satyam@infradead.org>
Cc: "Luck, Tony" <tony.luck@intel.com>
Cc: Stephane Eranian <eranian@hpl.hp.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Tony Luck <tony.luck@intel.com>