staging: ion: Remove left over comment

Commit 2bb9f503 ("gpu: ion: Remove heapmask from client")
removed the heap_type_mask parameter. Remove the associated
kernel-doc comment too.

Signed-off-by: Sachin Kamat <sachin.kamat@samsung.com>
Cc: Rebecca Schultz Zavin <rebecca@android.com>
Cc: John Stultz <john.stultz@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>