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>

cmake/compilers/gnu.cmake

@@ -8,7 +8,6 @@ enable_language(ASM)
 
 set(ASM_SOURCES
     ${SOURCE_DIR}/asm/ftola.c
-    ${SOURCE_DIR}/asm/matha.S
     ${SOURCE_DIR}/asm/snapvector.c
 )