Remove unused matha.S

Before commit a7d33b1c "(bug #3639) BoxOnPlaneSide rewrite" there
was more code here, but since then the only thing left is the
Invert24To16 function, to which I couldn't find any references since
the beginning of ioquake3's git history.

Resolves: https://github.com/ioquake/ioq3/issues/774
Signed-off-by: Simon McVittie <smcv@debian.org>

Makefile

@@ -2173,7 +2173,6 @@ endif
 ifeq ($(ARCH),x86)
   Q3OBJ += \
     $(B)/client/snd_mixa.o \
-    $(B)/client/matha.o \
     $(B)/client/snapvector.o \
     $(B)/client/ftola.o
 endif
@@ -2534,7 +2533,6 @@ Q3DOBJ = \
 
 ifeq ($(ARCH),x86)
   Q3DOBJ += \
-      $(B)/ded/matha.o \
       $(B)/ded/snapvector.o \
       $(B)/ded/ftola.o
 endif