spectral/sphere: simplify Makefile

By simply renaming the expected output files to follow the usual
conventions, we can simplify the `Makefile` a bit.
Signed-off-by: Michal Terepeta <michal.terepeta@gmail.com>

Test Plan: build & run

Reviewers: bgamari

Reviewed By: bgamari

Differential Revision: https://phabricator.haskell.org/D3366

spectral/sphere/Makefile

@@ -4,12 +4,6 @@ FAST_OPTS = 100
 NORM_OPTS = 100
 SLOW_OPTS = 200
 
-ifeq "$(mode)" "slow"
-SRC_RUNTEST_OPTS += -o1 spheres.slowppm
-else
-SRC_RUNTEST_OPTS += -o1 spheres.ppm
-endif
-
 include $(TOP)/mk/boilerplate.mk
 include $(TOP)/mk/target.mk